Computer Networks and Protocol

ระบบคอมพิวเตอร์ (Computer System)

2008-07-27T15:41:00.008+07:00

ระบบ (System) คือกลุ่มขององค์ประกอบที่มีความสัมพันธ์กันและทำงานร่วมกัน ซึ่งระบบคอมพิวเตอร์จะมีองค์ประกอบที่สำคัญ 3 ส่วน คือ
ฮาร์ดแวร์ (Hardware)
ซอฟต์แวร์ (Software)
บุคลากร (Peopleware)

ฮาร์ดแวร์ (Hardware) หมายถึง อุปกรณ์ต่าง ๆ ที่เป็นตัวเครื่องคอมพิวเตอร์ แบ่งออกเป็นส่วนประกอบดังนี้
หน่วยรับข้อมูล หน่วยประมวลผล หน่วยแสดงผล

1. หน่วยรับข้อมูล (Input unit) เป็นอุปกรณ์รับเข้า ทำหน้าที่รับโปรแกรมและข้อมูลเข้าสู่เครื่องคอมพิวเตอร์ อุปกรณ์รับเข้าที่ใช้กันเป็นส่วนใหญ่ คือ แป้นพิมพ์ ( Keyboard ) และเมาส์ ( Mouse) นอกจากนี้ยังมีอุปกรณ์รับเข้าอื่น ๆ อีก ได้แก่ สแกนเนอร์ ( Scanner), วีดีโอคาเมรา (Video Camera), ไมโครโฟน (Microphone),ทัชสกรีน (Touch screen), แทร็คบอล (Trackball), ดิจิตเซอร์ เทเบิ้ล แอนด์ ครอสแชร์ (Digiter tablet and crosshair)

2. หน่วยประมวลผลกลาง (Central Processing Unit) หรือเรียกโดยทั่ว ๆ ไปว่า CPU ซึ่งถือว่าเป็นสมองของระบบคอมพิวเตอร์ มีส่วนประกอบที่สำคัญ 2 ส่วน คือ หน่วยควบคุม หน่วยคำนวณ

หน่วยควบคุม (Control Unit หรือ CU) ทำหน้าที่ควบคุมลำดับขั้นตอนการทำงานของหน่วยรับข้อมูล หน่วยแสดงผล หน่วยคำนวณและหน่วยตรรก หน่วยความจำและแปลคำสั่ง
หน่วยคำนวณและตรรก (Arithmetic and Logic Unit หรือ ALU) ทำหน้าที่ในการคำนวณหาตัวเลข เช่น การบวก ลบ การเปรียบเทียบ
หน่วยความจำ เป็นอุปกรณ์ใช้เก็บโปรแกรมและข้อมูลที่ใช้ในการประมวลผล

3. หน่วยความจำภายใน (Primary Storage Section หรือ Memory) เป็นหน่วยความจำที่อยู่ภายในเครื่องคอมพิวเตอร์ที่สามารถติดต่อกับหน่วยงานอื่น ๆ ได้โดยตรง แบ่งออกเป็น 2 ประเภท

หน่วยความจำภายใน
- หน่วยความจำแบบแรม (Random Access Memory หรือ Ram) เป็นหน่วยความจำชั่วคราว ที่ใช้สำหรับเก็บโปรแกรมที่กำลังใช้งานอยู่ขณะนั้น มีความจุของหน่วยเก็บข้อมูลไม่เกิน 640 KB คือผู้ใช้สามารถเขียนหรือลบไปได้ตลอดเวลา ถ้าหากปิดเครื่องคอมพิวเตอร์หรือไฟฟ้าดับ จะมีผลทำให้ข้อมูลต่าง ๆ ที่เก็บไว้สูญหายไปหมด และไม่สามารถเรียกกลับคืนมาได้

- หน่วยความจำแบบรอม (Read Only Memory หรือ Rom) เป็นหน่วยความจำถาวร ที่สามารถอ่านได้อย่างเดียว ไม่สามารถบันทึกข้อมูลได้ ถึงแม้ว่าจะปิดเครื่องหรือไฟฟ้าดับ ข้อมูลที่เก็บไว้จะยังคงอยู่

2. หน่วยความจำสำรอง ได้แก่ เทปแม่เหล็ก จานแม่เหล็ก แผ่นดิสก์ (Diskett) CD-ROM
แผ่นดิสก์หรือสเกต เป็นจานแม่เหล็กขนาดเล็ก ชนิดอ่อน จัดเก็บข้อมูลโดยใช้อำนาจแม่เหล็ก การใช้งานจะต้องมี Disk Drive เพื่อใช้เป็นอุปกรณ์ในการขับเคลื่อนแผ่นดิสก์ โดยแบ่งตำแหน่งพื้นผิวออกเป็น แทร็คและเซ็คเตอร์ แบ่งออกเป็น 3 ขนาด คือ

แผ่นดิสก์ขนาด 8 นิ้ว ปัจจุบันไม่นิยมใช้
แผ่นดิสก์ขนาด 5.25 นิ้ว แบ่งออกเป็น DD สามรถบันทึกข้อมูลได้ประมาณ 360 KB และ HD สามารถบันทึกข้อมูลได้ 1.2 MB
แผ่นดิสก์ขนาด 3.5 นิ้ว แบ่งออกเป็น DD สามารถบันทึกข้อมูลได้ประมาณ 720 KB และ HD สามารถบันทึกข้อมูลได้ 1.44 MB นิยมใช้กันมากในปัจจุบัน

ขนาด 1.44 MB

ขนาด 5.25 นิ้ว

หน่วยความจำต่ำสุด คือ บิต (BIT [Binary Digit]) โดยใช้บิตแทน 1 ตัวอักขระ หรือ 1 ไบต์ (Bite) หน่วยที่ใหญ่ขึ้นมาอีกหน่วย คือ กิโลไบต์ (Kilobyte) โดยที่ 1 กิโลไบต์ มีค่าเท่ากับ 2 10 ไบต์ หรือ 1,024 ไบต์ หน่วยความจำที่ใหญ่ขึ้นไปอีก เรียกว่า เมกะไบต์ กิกะไบต์ และเทระไบต์

ฮาร์ดดิสก์ ( Hard Disk ) เป็นจานแม่เหล็กชนิดแข็ง ชนิดติดแน่นไม่มีการเคลื่อนที่ สามารถบรรจุข้อมูลได้จำนวนมาก เป็น 2 ขนาด คือ
1. ขนาด 5.25 นิ้ว (ปัจจุบันเลิกใช้แล้ว)
2. ขนาด 3.5 นิ้ว
ทั้ง 2 ขนาดจะมีความจุ ตั้งแต่ 10,20,40,80,120,300,400 MB1 GB,2 GB ฯลฯ ปัจจุบันนิยมใช้ตั้งแต่ 10 GB ขึ้นไป

Hard disk

Data Rate หมายถึง ความเร็วในการอ่านข้อมูลจากดิสก์ไปสู่สมองของเครื่องคอมพิวเตอร์ (หรือมีความเร็วในการนำข้อมูลมาจากสมองเครื่องไปบันทึกลงบนดิสก์) มีหน่วยวัดเป็น จำนวนไบต์ต่อวินาที ( Bytes Per Second หรือ bps )

ซีดีรอม (CD-Rom ) เป็นจานแสงชนิดหนึ่ง ใช้เก็บข้อมูลที่มีความเร็วในการใช้งานสูง มี

คุณสมบัติดังนี้

เป็นสือที่สามารถเก็บข้อมูลได้เป็นจำนวนมาก โดยจะมีความจุสูงถึง 2 GB (2 พันล้านไบต์)

มีขนาดเล็ก สามารถเคลื่อนย้ายได้สะดวก

ใช้เทคโนโลยีของแสงเลเซอร์ในการอ่านเขียนข้อมูล

เป็นจานแสงชนิดอ่านได้อย่างเดียว ( Read Only Memory ) ไม่สามารถเขียนหรือลบข้อมูลได้

CD - ROM

หน่วยแสดงผล (Output Unit) ทำหน้าที่แสดงผลลัพธ์ที่ได้จากการประมวลผลของเครื่องคอมพิวเตอร์ หรือใช้เก็บผลลัพธ์เพื่อนำไปใช้ภายหลัง ได้แก่ จอภาพ (Monitor) เป็นอุปกรณ์ส่งออกมากที่สุด เครื่องพิมพ์ (Printer)
ซอฟแวร์ (Software) หมายถึง โปรแกรมชุดคำสั่งที่เขียนให้เครื่องคอมพิวเตอร์ปฏิบัติตาม ซึ่งมี 2ประเภท คือ

ซอฟแวร์ควบคุมระบบ (System Software) คือ ชุดคำสั่งหรือโปรแกรมที่ควบคุมการทำงานของคอมพิวเตอร์ เป็นสื่อกลางระหว่างโปรแกรมประยุกต์กับเครื่องคอมพิวเตอร์ เพื่อช่วยในการจัดการทรัพยากรของคอมพิวเตอร์ ได้แก่ โปรแกรมควบคุมเครื่อง ระบบปฏิบัติการ เช่น DOS, Windows, Os/2, Unix
ซอฟแวร์ประยุกต์ (Application Software) คือ ชุดคำสั่งหรือโปรแกรมที่เขียนขึ้นมาเพื่อให้เครื่องคอมพิวเตอร์ทำงานตามที่ผู้ใช้ต้องการ ได้แก่ โปรแกรมสำเร็จรูปต่าง ๆ

บุคลากร (Peopleware) หมายถึง บุคลากรทางคอมพิวเตอร์ที่ทำหน้าที่ในการใช้และดูแลเครื่องคอมพิวเตอร์ เช่น นักเขียนโปรแกรม (Programmer) นักวิเคราะห์ระบบ (System Analyst) เป็นต้น

Very Easy Home Network

2008-07-02T22:55:00.000+07:00

Very Easy Home Network
By: Manbeer Singh

Ahhh yes, life’s pretty sweet with that high speed DSL line. But there is a glitch. The normal ISP dial up account can be used from any computer in the world. A DSL line connects to only one computer.

The obvious solution would be to install a network to carry the line to every computer in the house. I've steered clear of a having network in my home office. I once spent several days trying to hook up 2 computers using Windows 95, and finally gave up after only intermittently being able to make the computers communicate. I swore that my days on the floor with my computers were over and I ran my own "network" by transferring Zip disks from one computer to another.

A new technology is on the horizon, the Home PNA (Phone Networking Alliance) network. I remembered reading about this network that would connect through existing home phone lines. It was in its infancy - so I checked around.

HPNA has made some amazing strides. Now in version 2.0, it will run your home network at ethernet speed, 10Mbps (ten megabytes per second - the same speed as a real corporate network) and extend your high-speed internet connection to anywhere in your home or office. After checking out all the reviews, the same name kept coming up: Netgear.

Netgear offers a home PNA card that not only is plug and play, but also allows you to talk on the phone while using the network. Netgear's "do-not-disturb" feature causes it not to completely take over the phone line, nor will talking on the phone degrade the speed of the network connection.

The card was simple enough to install, just open the computer case and snap it into an available slot. Of course, this is usually the time that things go sour - when you start up the computer. Amazingly, "plug-and-play" was never easier. At first glance the instructions supplied on a single page fold-out sheet look like they’d never do the trick, but they do. Netgear includes a CD with all the software that you need. No need to search for the required drivers, their set-up program takes you step by step. Within a few minutes, you’ll be able to print a page on a remote computer’s printer or share files.

My teenage daughter installed the network on her own computer, and we can now share printers, files, a CD-burner and internet connections without using our "sneakernet" of the past.

My favorite part of the setup is the new Home Network USB adapter. I clipped the small unit to the back of my laptop, and literally within seconds, connected to my other computers!

If this sounds too easy, it probably is. I’m no techie and it worked for me on first try. One caveat though, when it comes to sharing a high speed internet connection over a Home Network line, there are a few more steps. You need to set up a TCP/IP connection. My advice? Don’t even attempt to do it unless you know what you’re doing. Call Netgear’s 24/7 tech support at 888-NETGEAR. They’ll patiently walk you through it, step by step.

I feel like I've really accomplished something, it's not "black-magic" any more. I have a flawless network that I never realized could be as useful as it is. I have a warp speed internet connection that goes beyond my wildest dreams. I passed the boundaries from novice to geek with the help of modern technology. Now my mornings are complete. I can stay that extra ½ hour in bed, watching Matt Lauer and Katie Couric on the Today show. No need to zoom into the office to check my auctions and email. I can just plug the laptop into the phone jack and fly like the wind.

Article Directory Source: http://www.1articleworld.com

Why Use Ethernet Routers

2008-07-02T22:53:00.001+07:00

By: Benjamin Brook

Ethernet routers are a key component of your home computer system if you have many devices that need to be connected to your computer. For example, printers, modems, other computers and game consoles. Having Ethernet routers at home is a good way to connect all these devices seamlessly into one machine and the Internet.

Usually you can connect up to four devices with Ethernet routers. Depending on how many devices you need connected to your computer and the Internet you can find a router, which allows more devices. Any device that you wish to connect Ethernet routers must have an Ethernet network adaptor.

Adding On

If you have looked at Ethernet routers that have less Ethernet connections than you need but think that such models fit into your budget you can simply adjust them. Using a network switch will allow you to expand the number of devices that you can connect.

Having a wireless access port is a handy thing to have; it will take up on of the access ports but at the same time will allow many wireless devices to work. The drawback is that other Wifi computers using the network at the same time will slow down the system significantly.

Advantages

Ethernet routers have an advantage over simple Ethernet cables in that you can connect more devices to your home network system. Ethernet cables restrict you to only two devices or two computers. If you find yourself with an increasing amount of devices that need to be connected to one central computer then Ethernet routers are the way to go.

You may also be tempted to use an ad hoc Wifi system instead of an Ethernet router. While this does allow you greater flexibility in terms of distance from the central system and the number of devices, it does have drawbacks. It is a less secure system and is usually used on a temporary basis. Ethernet routers are more secure.

For an ad hoc system to work properly all devices must have a Wifi network adaptor. You may also have to configure the adaptors to ad hoc mode as usually they are configured for the typical infrastructure mode. This might take up some extra time to set up and is a lot more complex than simple Ethernet routers.

If you have multiple computers that need to be connected it may be worth it to use an Ethernet switch or hub. The only disadvantage being that one computer must be connected to the Internet. The other computers will access the Internet through this central computer.

Article Directory Source: http://www.1articleworld.com

Cisco CBAC – The Poor Mans Firewall

2008-07-02T22:50:00.002+07:00

By: Nicholas Evra

CBAC Overview
The Cisco IOS Firewall Feature Set is a module that can be added to the existing IOS to provide firewall functionality without the need for hardware upgrades. There are two components to the Cisco IOS Firewall Feature Set in Intrusion Detection (which is an optional bolt-on) and Context-Based Access Control (CBAC). CBAC maintains a state table for all of the outbound connections on a Cisco router by inspecting tcp and udp connections at layer seven of the OSI model and populating the table accordingly. When return traffic is received on the external interface it is compared against the state table to see if the connection was originally established from within the internal network, and then either permitted or denied. Although basic this is a very effective mechanism to prevent unauthorized access to the internal network from external sources such as the internet.

CBAC Application-specific support

Cisco have also built in some additional functionality into CBAC in terms of application-specific inspection that enables the router to recognize and identify application specific data flows such as HTTP, SMTP, TFTP, and FTP. Understanding these applications and their data flows empowers the router to identify malformed packets or suspect application data flows and permit or deny accordingly. CBAC also provides the flexibility of downloading Java code from trusted sites, but it denying untrusted sites.

CBAC and Denial of Service (DOS) Attacks

Denial-Of-Service (DOS) attack protection is also in-built with real-time logging of alerts as well as pro-active responses to mitigate the threat. To do this CBAC can be configured to manage half-open TCP connections which are used in TCP SYN flood attacks to overload a targets resources resulting in a denial of service to legitimate users. To do this CBAC uses timeouts and thresholds, which are configurable, to determine how long state information for each connection should be kept for sessions and when to drop them. Note that UDP and ICMP require that an idle-timer limit is used to determine when a connection should be terminated. A very useful command to identify a DOS attack is ‘ip inspect audit-trail’ which logs all DOS connections including source and destination IP address and TCP or UDP ports allowing you to pin-point the exact source and destination of the attack.

Configuring CBAC

There are five steps to configuring CBAC on a Cisco router in order for it to function correctly. These are as follows:
1. Choose an interface to which inspection will be applied. This can be an internal or external interface as CBAC is only concerned with the direction of the first packet initiating the connection which is identified when applying CBAC to an interface.

2. Configure an IP access list in the correct direction on the selected interface to allow traffic through for CBAC to inspect.

3. Configure global timeouts and thresholds for established connections or sessions.

4. Define an inspection rule specifying exactly which protocols will be inspected by CBAC.

5. Apply the inspection rule to the interface in the correct direction.

Article Directory Source: http://www.1articleworld.com

Home Computer Networking

2008-07-02T22:46:00.002+07:00

By: travis klein

Numbers of people these days have two computers in their homes; one is usually kept in the children's room and another in their personal room. So its important for you to know about home networking and its benefits that you can have from both systems in your home. But for this you should know certain things like how to set up home networking, what all you will need for it etc.

In order to make home network set-up easy and useful, you need to install a network friendly OS (operating system) like Windows ME or XP. But this again is not very important; it all depends upon your requirement. Further you will need many more items to make home networking successful. They are:

Two Network cards - If your computer is quite new, ask the shopkeeper for two PCI network cards of 10/100mbps specification.

RJ-45 crossover cable - This is the cable needed to join the machines together so ensure that the length is long enough.

Basic knowledge of using a Windows PC. We will discuss advantages of Home Networking now. Here are some of the benefits that you will get through home networking:

You can play various games across the network say with your kid in another room. Also, you will be able to share the pieces of hardware. For instance, if you have single printer but two computers, you could use the same printer for both computers.

If you have lots of files in any one computer and want both the systems to have access over it, it will be possible using a home network.

Steps for Home Networking

1. The first step is to take the cover/lid out of both the computers and then fit the Network cards into a free PCI slot. Screw them down into the case and close the lid.

2. After restarting the computer, Windows should reappear mentioning about the finding of a new hardware and will ask for a driver disk. Then you need to insert the CD into the drive and wait for the drivers to get installed. Do the same with second computer and thus computers are enabled to communicate after this step.

3. Now, click on Start, then Settings, Control Panel and Network Connections and finally click at Set up a home network. Then you will be asked several questions and the system will ask you to insert a floppy disk into the computer, repeat the same with second computer.

Your home network base is installed so start doing what you want. If you face major problems take help of professionals and for minor difficulties just click on Help option for assistance.

Article Directory Source: http://www.1articleworld.com

Networking (Computers)

2008-07-02T22:42:00.002+07:00

By: Christoff Genviere

There are many kinds of networks, however this paper will be about networking computers. As we move further and further into the paperless society, the need for people to be connected and able to exchange data just as fast as they could by handing a paper to someone increases. This can be accomplished by having a group of computers connected by a network, so that as soon as data is entered into one computer, it can be immediately accessed by someone else on a connected computer, no matter how far away it may be (though usually it is in the same building). There is much work involved in this and it in includes a lot of math, from equations to basic problems. This report will be based around the mathematical aspects of setting up a network.

The first mathematical question in setting up a network is very basic. How many computers will be connected to this network and how many guest computers might come on at one time is the question. An example of a guest computer is if someone brought a laptop and connected it for a short while to download or access data. To find the answer to the question, simply count the desktop computers that will be connected and how many guest computers you expect to be connected at one time.

The second mathematical problem that occurs is best solved using an algebraic equation. Let x=the amount of desktop computers that will always be connected, y=the amount of guest computers that you expect to be connected at one time. So, the equation is: x+y+1. The one added on the end of the equation is another guest file just to make sure you don't fall short. So, this tells you how many files you need to create. The guest files will all be generically named so that all guests have the same access privileges, and all the permanent computers will have their own named file so they can have more personalized access privileges. These files are put on one main computer, the server. This controls all access privileges and any data put into a computer branching off from it in it's network can be accessed from this all-powerful server computer. The previously stated problems are a large part of networking, although I couldn't possibly tell about all the math involved without going on for another 3 or 4 pages. Those problems help with networking as far as setting up the network on the computer goes, but there is a whole nother side. The physical side.

The physical side of computer networking involves problems such as how many feet of cable are you going to need to connect the computers. Some large office buildings can have 1 mile of cable between their networked computers! If someone has 2 computers in their house, it may only involve 3 feet. The mathematical procedure is quite simple although it might take a while to complete. Just take out the old meter stick and start measuring. Don't measure direct lines between the computers unless you want the cable stretching in a straight line between them. Chances are you will want it to run along a wall or around another object. Once the measuring is done, just add up the cable length and you have the answer to the problem.

If you don't have a very tight budget, you can afford faster networks than cable networks. These are more sophisticated but I was lucky enough to get to try it this summer. It is called infrared data transfer (IDT). Instead of cables, you have an infrared connector hooked to your computer. Just aim the little infrared panel at the infrared panel on the other computer and it will trade information with infrared light. These panels are usually about 1 square inch in size.

This is much quicker and doesn't involve annoying cables. You still need to gauge distance because there is a distance limit on how far apart they can be and still work. When you install them, your computer will ask you questions such as how many lumens (measurement of brightness of light) you would like your panel to emit.

It is invisible to the naked eye but the amount of lumens it outputs is critical. If you have a fast computer, you might want more lumens so that your computer doesn't crash because of lagging. If you have a slower computer you will want less lumens because other wise you will be sending data too fast for your computer and there will end up being a lot of gibberish that will mess up the receiving computer.

The mathematical things that networking involves are almost endless depending on the situation. I couldn't adequately explain them if I had the time, because anybody who didn't understand quite a bit about computers wouldn't know what I was talking about, forcing me to explain many things that are off the subject of mathematics in networking. I hope I have given you an idea of what it involves, but if you want to know about all of the mathematics, you will just have to network some computers yourself.

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Why Use Ethernet Routers

2008-07-02T21:41:00.001+07:00

By: Benjamin Brook

Ethernet routers are a key component of your home computer system if you have many devices that need to be connected to your computer. For example, printers, modems, other computers and game consoles. Having Ethernet routers at home is a good way to connect all these devices seamlessly into one machine and the Internet.

Usually you can connect up to four devices with Ethernet routers. Depending on how many devices you need connected to your computer and the Internet you can find a router, which allows more devices. Any device that you wish to connect Ethernet routers must have an Ethernet network adaptor.

Adding On
If you have looked at Ethernet routers that have less Ethernet connections than you need but think that such models fit into your budget you can simply adjust them. Using a network switch will allow you to expand the number of devices that you can connect.

Having a wireless access port is a handy thing to have; it will take up on of the access ports but at the same time will allow many wireless devices to work. The drawback is that other Wifi computers using the network at the same time will slow down the system significantly.

Advantages
Ethernet routers have an advantage over simple Ethernet cables in that you can connect more devices to your home network system. Ethernet cables restrict you to only two devices or two computers. If you find yourself with an increasing amount of devices that need to be connected to one central computer then Ethernet routers are the way to go.

You may also be tempted to use an ad hoc Wifi system instead of an Ethernet router. While this does allow you greater flexibility in terms of distance from the central system and the number of devices, it does have drawbacks. It is a less secure system and is usually used on a temporary basis. Ethernet routers are more secure.

For an ad hoc system to work properly all devices must have a Wifi network adaptor. You may also have to configure the adaptors to ad hoc mode as usually they are configured for the typical infrastructure mode. This might take up some extra time to set up and is a lot more complex than simple Ethernet routers.

If you have multiple computers that need to be connected it may be worth it to use an Ethernet switch or hub. The only disadvantage being that one computer must be connected to the Internet. The other computers will access the Internet through this central computer.

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For more information about routers please visit my website cordless phones and wireless routers problems

IP Addresses, Subnetting and Sub-subnetting

2008-06-15T22:55:00.002+07:00

This page will give you a basic understanding of the structure of IP addresses and subnets as well as specific information about sub-subnetting at Cornell. We recommend that you read through the entire page; however, if you're familiar with these issues, use the table of contents to click ahead to the section you're most interested in.

Contents of this page:
What are IP addresses and how are they used?
IP Addressing Space
How does Sub-subnetting work?
Understanding Static vs. Dynamic Addressing
Changing the UNIX Operating System to Work with Sub-subnetting
Host registration

What Are IP Addresses and How Are They Used?

IP (Internet Protocol) addresses are used to identify hosts on the campus Internet, a Cornell network that ties into the Internet, a global network. If the computer is attached to Cornell's network, it needs an IP address to be recognized as part of the campus Internet.

IP addresses are constructed according to a set of specific rules so that hosts on any part of the Internet can communicate with each other. This document describes IP addresses only as they apply to Cornell's campus network. (If you want to know more about Internet addressing, refer to Internetworking with TCP/IP: Principles, Protocols, and Architecture by Douglas Comer, Prentice Hall).

An IP address consists of a 32-bit binary number, which is typically presented as four decimal numbers (one for each 8-bit byte) separated by decimal points. For example, 128.253.21.58.

Internet addresses at Cornell have three parts:

network address
subnet address
host address

When you configure a host for sub-subnetting, you are primarily concerned about the host address, but some understanding of the network address and subnet address is useful.

Network Address
Cornell has four addresses for its backbone networks. They are 128.253.0.0, 128.84.0.0, 132.236.0.0, and 140.251.0.0. The latter is used only by the Cornell University College. These addresses are assigned to Cornell. Cornell cannot change the first two parts of each address, but is free to use the last two parts in any way it chooses in order to identify Local Area Networks (subnets) and hosts that are connected to the campus Internet.

Subnet Address

The subnet address is the address given to your Local Area Network (LAN). Cornell's system provides for 254 LANs connected to each of the main networks. So, for example, if your LAN is identified on the network as 128.253.0.0, a possible subnet addresses (or LAN address) might be 128.253.21.0. The third number, 21, identifies the subnet.

Host Address

The host address is the address given to the workstation, other computer, or device that is connected to the LAN. Cornell's system provides for 256 host addresses on each LAN. So, for example, if your host is identified on the LAN as 128.253.21.0 a possible host address is 128.253.21.58. The last number, 58, identifies the host.

Not all 256 numbers are available as host addresses on any given LAN. Zero (0) and 255 are reserved for broadcast purposes. (Hosts are set up to "grab" any message marked with their own address or a broadcast address; for example, if your host address is 128.253.21.58 and it "sees" a message addressed to 128.253.21.255, it will grab the message. In this way, hosts can send messages to large groups without having to know each address on their LAN.)

One (1) is reserved for the gateway/router that sits between the LAN and next network level. The numbers 2-5 are reserved by CIT for diagnostic and management use.

IP Addressing Space

This addressing scheme has worked well for Cornell, but it has some limitations:

Each of Cornell's fiber backbones can have no more than 256 LANs attached to them.
Each LAN can have no more than 256 (249 if reserved addresses are taken into account) hosts. Most LANs are constructed with far fewer than the maximum number of hosts addresses available.
Ethernet LANs performance is reduced with a large number (100 or more) of connections. Performance is most affected by how people are using the LAN: a small number of heavy users can bog down the performance of any LAN. Therefore, the limitations on the number of host addresses hasn't been and isn't expected to be a problem for most LANs at Cornell.

To make room for additional subnet addresses, or LANs, CIT has used a system called sub-subnetting. With this system, up to four LANs can use one full subnet address, thus effectively quadrupling the number of subnet addresses available.

How Does Sub-Subnetting Work?
Subnetting

Each subnet address at Cornell is assigned a "subnet mask." A subnet mask defines how many bits are used for the network address and how many for the host address.

The subnet mask address is 255.255.255.0, and it currently is the same for all LANs. If you convert the subnet mask address to its binary form, it looks like this:

Subnet mask: 11111111 11111111 11111111 00000000

If you convert our example host address (128.253.21.58) to its binary form, it looks like this:

Host address: 10000000 11111101 00010101 00111010

Together they look like this:

Subnet mask: 11111111 11111111 11111111 00000000
Host address: 10000000 11111101 00010101 00111010

The subnet mask when shown this way, as an overlay on the host address, essentially tells the computer which part of the IP address is a network address and which part is a host address. Everything in the host address that corresponds to a 1 in the subnet mask is a network address and everything in the host address that corresponds to a 0 in the subnet mask is a host address.

Sub-subnetting

Many LANs at Cornell are sub-subnetted.

Sub-subnetting is based on the same concept as subnetting. With sub-subnetting, the mask will be 255.255.255.192. In binary form, this address looks like this:

Sub-subnet mask: 11111111 11111111 11111111 11000000

When you compare the sub-subnet mask to the example IP host address (128.253.21.58), they look like this:

Sub-subnet mask: 11111111 11111111 11111111 11000000
Host address: 10000000 11111101 00010101 00111010
Like the old subnet mask, the sub-subnet mask tells the computer which part of the IP address is the network address and which part is a host address. As explained above, everything in the host address that corresponds to a "1" in the sub-subnet mask is a network address and everything in the host address that corresponds to a 0 in the sub-subnet mask is a host address.
As you can see, there are now ones (1's) in the last byte of the sub-subnet mask. (This is the part, in the subnet mask, that was all zeros (0's) and that identified the host portion of an address.) The ones (1's) that appear in this byte of the sub-subnet mask identify the first two bits of the last byte of an IP address as part of the network portion of the address. The remaining zeros (0's) identify the host portion of the address. By increasing the number of bits assigned to network addressing, the number of possible network addresses increases and the number of possible host addresses for each LAN decreases.

Why is this important?

Gateways need to forward packets to other gateways to get them to the destination LAN. Each network interface card on the gateway is assigned an IP address and a sub-subnet mask. This enables the gateways to route packets from one LAN to another LAN. Once the packet arrives at a gateway that is attached to the destination LAN, the gateway then uses the two bits of the sub-subnet portion of the IP address (the first two bits of the last byte of the IP address) to decide to which sub-subnetted LAN to send the packet.

In order for this to work, each of the LANs connected to a given gateway must have a different set of host addresses. As you know, the host address you assign, for example 58, is translated into a binary address (00111010). Remember, even though you think of this as a host address, the sub-subnet mask forces the gateway to think of the first two bits as part of the network address. If two LANs attached to the same gateway each have hosts with an address of 58, the first two bits in the binary translation will be the same, and even though the hosts are physically on two separate LANs, the gateway won't know to which LAN to send a packet.

To make sure each LAN has a unique set of host addresses, network administrators can no longer assign host numbers from the entire range. Now, each administrator will be assigned a subset of the host addresses available. Within each subset, broadcast, gateway, and diagnostic addresses must be reserved. So, for example, Administrator X can have server addresses at 6 through 9 and workstation addresses at 10 through 63; Administrator Y can have server addresses at 70 through 73 and workstation addresses 74 through 126, etc. If your current host addresses fall outside the range assigned to your LAN, you need to reassign host addresses on your network.

Understanding Static vs. Dynamic Addressing
If you use static addressing on your network, this means you assign each host a permanent IP address. If you use dynamic addressing, the hosts use any available address within a range you specify. The information below explains some of the advantages and disadvantages of static and dynamic addressing.

Before you begin reconfiguring your applications, you should decide which scheme you plan to use: static, dynamic, or a combination. CIT recommends that you use static addressing; it helps you, as the network administrator, keep track of machines and figure out which one is causing problems. Some network applications require static addressing.

If you want to consider dynamic addressing because it is easier to set up, CIT advises that you limit the range for dynamically assigned addresses to start at 21, thus leaving 6-20 reserved for hosts that need fixed addresses because they will offer IP services. If you think you will have more than 15 IP server hosts, you should raise the start point for dynamically assigned addresses.

Changing the UNIX Operating System to Work with Sub-subnetting

Because UNIX has TCP/IP protocols built into the operating system, you can alter the configuration of the operating system itself instead of the applications that run on it. Follow these instructions:

Go to the interface configuration command, called ifconfig, in your startup script. Most UNIX startup scripts are called /etc/rc/local. You will see a line that looks like this:ifconfig le0 $hostname netmask 255.255.255.0 broadcast 128.253.180.255 -trailers up
Change the value of the netmask to 255.255.255.192.
Note: ifconfig also accepts netmask values in hexadecimal notation. Therefore you may see 0xffffff00 as the value of the netmask. If this is the case, change the value to 0xffffffc0.
Change the host portion of the broadcast address to one of the following values, depending on the sub-subnet your host is in:

If the host's IP address is between 1 and 64, use 63.
If the host's IP address is between 65 and 128, use 127.
If the host's IP address is between 129 and 192, use 191.
If the host's IP address is between 193 and 256, use 255.

If your IP address is between 1 and 64, on subnet 180, your ifconfig command would look like this: ifconfig le0 $hostname netmask 255.255.255.192 broadcast 128.253.180.63 -trailers up

4. If you use static routing, you need to change your default gateway. For example:
route add default 128.253.180.1 1

Host Registration
Network administrators are strongly encouraged to register all devices on their networks (desktop workstations, servers, printers, etc.) with the Network Operations Center (NOC). Visit the Network and Host Registration web site for more information.

IP Address Classes

2008-06-15T22:51:00.002+07:00

The original IP addressing design was based on Address Classes.

In the original Internet routing scheme developed in the 1970s, sites were assigned addresses from one of three classes: Class A, Class B and Class C. The address classes differ in size and number. Class A addresses are the largest, but there are few of them. Class Cs are the smallest, but they are numerous. Classes D and E are also defined, but not used in normal operation.

To say that class-based IP addressing in still used would be true only in the loosest sense. Many addressing designs are still class-based, but an increasing number can only be explained using the more general concept of CIDR, which is backwards compatible with address classes.

Suffice it to say that at one point in time, you could request the Internet NIC to assign you a class A, B or C address. To get the larger class B addresses, you might have to supply some justification, but only the class A was really tough to get. In any case, NIC would set the network bits, or n-bits, to some unique value and inform the local network engineer. It would then be up to the engineer to assign each of his hosts an IP address starting with the assigned n-bits, followed by host bits, or h-bits, to make the address unique.

Internet routing used to work like this: A router receiving an IP packet extracted its Destination Address, which was classified (literally) by examining its first one to four bits. Once the address's class had been determined, it was broken down into network and host bits. Routers ignored the host bits, and only needed to match the network bits to find a route to the network. Once a packet reached its target network, its host field was examined for final delivery.

Summary of IP Address Classes

Class A - 0nnnnnnn hhhhhhhh hhhhhhhh hhhhhhhh

First bit 0; 7 network bits; 24 host bits
Initial byte: 0 - 127
126 Class As exist (0 and 127 are reserved)
16,777,214 hosts on each Class A

Class B - 10nnnnnn nnnnnnnn hhhhhhhh hhhhhhhh

First two bits 10; 14 network bits; 16 host bits
Initial byte: 128 - 191
16,384 Class Bs exist
65,532 hosts on each Class B

Class C - 110nnnnn nnnnnnnn nnnnnnnn hhhhhhhh

First three bits 110; 21 network bits; 8 host bits
Initial byte: 192 - 223
2,097,152 Class Cs exist
254 hosts on each Class C

Class D - 1110mmmm mmmmmmmm mmmmmmmm mmmmmmmm

First four bits 1110; 28 multicast address bits
Initial byte: 224 - 247
Class Ds are multicast addresses - see RFC 1112

Class E - 1111rrrr rrrrrrrr rrrrrrrr rrrrrrrr

First four bits 1111; 28 reserved address bits
Initial byte: 248 - 255
Reserved for experimental use

Network Design Manual

2008-06-15T22:41:00.003+07:00

IP 101: All About IP Addresses

By Chris Lewis The key to understanding IP, and all of the issues related to IP, is knowing what a routing table looks like and the effects each IP topic has on the entries in a routing table. To begin with, let's review the basics. IP addresses are 32 bit numbers, most commonly represented in dotted decimal notation (xxx.xxx.xxx.xxx). Each decimal number represents eight bits of binary data, and therefore can have a decimal value between 0 and 255. IP addresses most commonly come as class A, B, or C. It's the value of the first number of the IP address that determines the class to which a given IP address belongs. Class D addresses are used for multi-cast applications.

(For a full explanation of class D addresses, refer to "Diving Through the Layers" .) The range of values for these classes are given below.

Class Range Allocation

A 1-126 N.H.H.H

B 128-191 N.N.H.H

C 192-223 N.N.N.H

D 224-239 Not applicable

N=Network
H=Host

Note 1: 127.0.0.0 is a class A network, but is reserved for use as a loopback address (typically 127.0.0.1).

Note 2: The 0.0.0.0 network is reserved for use as the default route.

Note 3: Class D addresses are used by groups of hosts or routers that share a common characteristic: e.g. all OSPF devices respond to packets sent to address 224.0.0.2

Note 4: Class E addresses exist (240-248),

but are reserved for future use

The class of an address defines which portion of the address identifies the Network number and which portion identifies the Host, as illustrated above, as N and H.

So, without any subnetting (which we will come to a little later), a routing table will keep track of a) network numbers, b) the next hop router to use to get to that network, and c) the interface this next hop router is reachable through. A simple network with the corresponding routing table for a Cisco router is illustrated below.

C 199.2.2.0 directly connected Ethernet 0
C 10.0.0.0 directly connected Token-ring 1
C 152.8.0.0 directly connected Ethernet 1
I 200.1.1.0 via 152.8.1.2 Ethernet 1

Since Cisco doesn't give headings for these columns, you need to know what each column consists of. The first column of the routing table indicates how the network number was discovered. C stands for Connected and I indicates the network was learned from the IGRP routing protocol. For a full description of the routing table as it appears in a UNIX host and a Cisco router, refer to "Should RIP Rest In Peace" .

The important thing to realize is that while a routing table keeps track of network numbers, no one assigns a network number to any piece of equipment. Every interface of a router or host connected on the network must have an IP address and a subnet mask defined (many pieces of equipment will assign a default subnet mask if none is applied). From this IP address and subnet mask, the network number is derived by the IP stack and tracked in the routing table.

(This is the exact opposite of what happens in a NetWare network. In NetWare, you assign a network number to a server LAN card, which is used by all workstations on that wire. The workstations use MAC addresses as IPX node numbers.)

Routing tables can get very large. Internet backbone routers can have over 40,000 routes defined in them. In most corporate networks, the routing table is much smaller, as there are not so many subnets that need to be reached.

Many large routers, particulary internet routers, use a method called Classless Interdomain Routing (CIDR) to reduce the number of entries a router needs in its routing table. If we imagine, for instance, that all the Class C addresses that start with the value 194 are allocated for use in Europe, it would significantly reduce the number of entries in Internet routers in the US if there was only one entry for all these class C addresses, rather than a separate entry in the routing table for each one. CIDR works if (as in this example) all the networks with the first octet value of 194 are physically located in one area of the network.

IP addresses are used to deliver packets of data across a network and have what is termed end-to-end significance. This means that the source and destination IP address remains constant as the packet traverses a network. Each time a packet travels through a router, the router will reference it's routing table to see if it can match the network number of the destination IP address with an entry in its routing table. If a match is found, the packet is forwarded to the next hop router for the destination network in question (note that a router does not necessarily know the complete path from source to destination--it just knows the next hop router to go to). If a match is not f ound, one of two things happens. The packet may be forwarded to the router defined as the default gateway, or the packet may be dropped by the router. (In the language of TCP/IP, a gateway is a router.)

Packets are forwarded to a default router in the belief that the default router has more network information in its routing table and will therefore be able to route the packet correctly on to its final destination. This is typically used when connecting a LAN with PCs on it to the Internet. Each PC will have the router that connects the LAN to the Internet defined as its default gateway.

A default gateway is seen in a routing table of a host as follows: the default route 0.0.0.0 will be listed as the destination network, and the IP address of the default gateway will be listed as the next hop router.

If the source and destination IP addresses remain constant as the packet works its way through the network, how is the next hop router addressed? In a LAN environment this is handled by the MAC (Media Access Control) address, as illustrated below. The key point is that the MAC addresses will change every time a packet travels though a router, however, the IP addresses will remain constant.

PC1 Router E0 Router E1 PC2
MAC Address M1 M2 M3 M4
Software (IP) address 11 12 13 14
A packet sent from PC1 to PC2 will look like this at point A:
Destination Source Destination Source Data
MAC MAC IP IP
M2 M1 14 11 1001001
A packet sent from PC1 to PC2 will look like this at point B:
Destination Source Destination Source Data
MAC MAC IP IP
M4 M3 14 11 1001001

Introduction to TCP/IP

2008-06-15T21:52:00.004+07:00

Introduction to TCP/IP

Summary: TCP and IP were developed by a Department of Defense (DOD) research project to connect a number different networks designed by different vendors into a network of networks (the "Internet"). It was initially successful because it delivered a few basic services that everyone needs (file transfer, electronic mail, remote logon) across a very large number of client and server systems. Several computers in a small department can use TCP/IP (along with other protocols) on a single LAN. The IP component provides routing from the department to the enterprise network, then to regional networks, and finally to the global Internet. On the battlefield a communications network will sustain damage, so the DOD designed TCP/IP to be robust and automatically recover from any node or phone line failure. This design allows the construction of very large networks with less central management. However, because of the automatic recovery, network problems can go undiagnosed and uncorrected for long periods of time.

As with all other communications protocol, TCP/IP is composed of layers:

IP - is responsible for moving packet of data from node to node. IP forwards each packet based on a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments. IP operates on gateway machines that move data from department to organization to region and then around the world.

TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.
Sockets - is a name given to the package of subroutines that provide access to TCP/IP on most systems.

Network of Lowest Bidders

The Army puts out a bid on a computer and DEC wins the bid. The Air Force puts out a bid and IBM wins. The Navy bid is won by Unisys. Then the President decides to invade Grenada and the armed forces discover that their computers cannot talk to each other. The DOD must build a "network" out of systems each of which, by law, was delivered by the lowest bidder on a single contract.

The Internet Protocol was developed to create a Network of Networks (the "Internet"). Individual machines are first connected to a LAN (Ethernet or Token Ring). TCP/IP shares the LAN with other uses (a Novell file server, Windows for Workgroups peer systems). One device provides the TCP/IP connection between the LAN and the rest of the world.

To insure that all types of systems from all vendors can communicate, TCP/IP is absolutely standardized on the LAN. However, larger networks based on long distances and phone lines are more volatile. In the US, many large corporations would wish to reuse large internal networks based on IBM's SNA. In Europe, the national phone companies traditionally standardize on X.25. However, the sudden explosion of high speed microprocessors, fiber optics, and digital phone systems has created a burst of new options: ISDN, frame relay, FDDI, Asynchronous Transfer Mode (ATM). New technologies arise and become obsolete within a few years. With cable TV and phone companies competing to build the National Information Superhighway, no single standard can govern citywide, nationwide, or worldwide communications.

The original design of TCP/IP as a Network of Networks fits nicely within the current technological uncertainty. TCP/IP data can be sent across a LAN, or it can be carried within an internal corporate SNA network, or it can piggyback on the cable TV service. Furthermore, machines connected to any of these networks can communicate to any other network through gateways supplied by the network vendor.

Addresses

Each technology has its own convention for transmitting messages between two machines within the same network. On a LAN, messages are sent between machines by supplying the six byte unique identifier (the "MAC" address). In an SNA network, every machine has Logical Units with their own network address. DECNET, Appletalk, and Novell IPX all have a scheme for assigning numbers to each local network and to each workstation attached to the network.

On top of these local or vendor specific network addresses, TCP/IP assigns a unique number to every workstation in the world. This "IP number" is a four byte value that, by convention, is expressed by converting each byte into a decimal number (0 to 255) and separating the bytes with a period. For example, the PC Lube and Tune server is 130.132.59.234.

An organization begins by sending electronic mail to Hostmaster@INTERNIC.NET requesting assignment of a network number. It is still possible for almost anyone to get assignment of a number for a small "Class C" network in which the first three bytes identify the network and the last byte identifies the individual computer. The author followed this procedure and was assigned the numbers 192.35.91.* for a network of computers at his house. Larger organizations can get a "Class B" network where the first two bytes identify the network and the last two bytes identify each of up to 64 thousand individual workstations. Yale's Class B network is 130.132, so all computers with IP address 130.132.*.* are connected through Yale.

The organization then connects to the Internet through one of a dozen regional or specialized network suppliers. The network vendor is given the subscriber network number and adds it to the routing configuration in its own machines and those of the other major network suppliers.

There is no mathematical formula that translates the numbers 192.35.91 or 130.132 into "Yale University" or "New Haven, CT." The machines that manage large regional networks or the central Internet routers managed by the National Science Foundation can only locate these networks by looking each network number up in a table. There are potentially thousands of Class B networks, and millions of Class C networks, but computer memory costs are low, so the tables are reasonable. Customers that connect to the Internet, even customers as large as IBM, do not need to maintain any information on other networks. They send all external data to the regional carrier to which they subscribe, and the regional carrier maintains the tables and does the appropriate routing.

New Haven is in a border state, split 50-50 between the Yankees and the Red Sox. In this spirit, Yale recently switched its connection from the Middle Atlantic regional network to the New England carrier. When the switch occurred, tables in the other regional areas and in the national spine had to be updated, so that traffic for 130.132 was routed through Boston instead of New Jersey. The large network carriers handle the paperwork and can perform such a switch given sufficient notice. During a conversion period, the university was connected to both networks so that messages could arrive through either path.

Subnets

Although the individual subscribers do not need to tabulate network numbers or provide explicit routing, it is convenient for most Class B networks to be internally managed as a much smaller and simpler version of the larger network organizations. It is common to subdivide the two bytes available for internal assignment into a one byte department number and a one byte workstation ID.

The enterprise network is built using commercially available TCP/IP router boxes. Each router has small tables with 255 entries to translate the one byte department number into selection of a destination Ethernet connected to one of the routers. Messages to the PC Lube and Tune server (130.132.59.234) are sent through the national and New England regional networks based on the 130.132 part of the number. Arriving at Yale, the 59 department ID selects an Ethernet connector in the C& IS building. The 234 selects a particular workstation on that LAN. The Yale network must be updated as new Ethernets and departments are added, but it is not effected by changes outside the university or the movement of machines within the department.
A Uncertain Path

Every time a message arrives at an IP router, it makes an individual decision about where to send it next. There is concept of a session with a preselected path for all traffic. Consider a company with facilities in New York, Los Angeles, Chicago and Atlanta. It could build a network from four phone lines forming a loop (NY to Chicago to LA to Atlanta to NY). A message arriving at the NY router could go to LA via either Chicago or Atlanta. The reply could come back the other way.

How does the router make a decision between routes? There is no correct answer. Traffic could be routed by the "clockwise" algorithm (go NY to Atlanta, LA to Chicago). The routers could alternate, sending one message to Atlanta and the next to Chicago. More sophisticated routing measures traffic patterns and sends data through the least busy link.

If one phone line in this network breaks down, traffic can still reach its destination through a roundabout path. After losing the NY to Chicago line, data can be sent NY to Atlanta to LA to Chicago. This provides continued service though with degraded performance. This kind of recovery is the primary design feature of IP. The loss of the line is immediately detected by the routers in NY and Chicago, but somehow this information must be sent to the other nodes. Otherwise, LA could continue to send NY messages through Chicago, where they arrive at a "dead end." Each network adopts some Router Protocol which periodically updates the routing tables throughout the network with information about changes in route status.

If the size of the network grows, then the complexity of the routing updates will increase as will the cost of transmitting them. Building a single network that covers the entire US would be unreasonably complicated. Fortunately, the Internet is designed as a Network of Networks. This means that loops and redundancy are built into each regional carrier. The regional network handles its own problems and reroutes messages internally. Its Router Protocol updates the tables in its own routers, but no routing updates need to propagate from a regional carrier to the NSF spine or to the other regions (unless, of course, a subscriber switches permanently from one region to another).

Undiagnosed Problems

IBM designs its SNA networks to be centrally managed. If any error occurs, it is reported to the network authorities. By design, any error is a problem that should be corrected or repaired. IP networks, however, were designed to be robust. In battlefield conditions, the loss of a node or line is a normal circumstance. Casualties can be sorted out later on, but the network must stay up. So IP networks are robust. They automatically (and silently) reconfigure themselves when something goes wrong. If there is enough redundancy built into the system, then communication is maintained.

In 1975 when SNA was designed, such redundancy would be prohibitively expensive, or it might have been argued that only the Defense Department could afford it. Today, however, simple routers cost no more than a PC. However, the TCP/IP design that, "Errors are normal and can be largely ignored," produces problems of its own.

Data traffic is frequently organized around "hubs," much like airline traffic. One could imagine an IP router in Atlanta routing messages for smaller cities throughout the Southeast. The problem is that data arrives without a reservation. Airline companies experience the problem around major events, like the Super Bowl. Just before the game, everyone wants to fly into the city.

After the game, everyone wants to fly out. Imbalance occurs on the network when something new gets advertised. Adam Curry announced the server at "mtv.com" and his regional carrier was swamped with traffic the next day. The problem is that messages come in from the entire world over high speed lines, but they go out to mtv.com over what was then a slow speed phone line.

Occasionally a snow storm cancels flights and airports fill up with stranded passengers. Many go off to hotels in town. When data arrives at a congested router, there is no place to send the overflow. Excess packets are simply discarded. It becomes the responsibility of the sender to retry the data a few seconds later and to persist until it finally gets through. This recovery is provided by the TCP component of the Internet protocol.

TCP was designed to recover from node or line failures where the network propagates routing table changes to all router nodes. Since the update takes some time, TCP is slow to initiate recovery. The TCP algorithms are not tuned to optimally handle packet loss due to traffic congestion. Instead, the traditional Internet response to traffic problems has been to increase the speed of lines and equipment in order to say ahead of growth in demand.

TCP treats the data as a stream of bytes. It logically assigns a sequence number to each byte. The TCP packet has a header that says, in effect, "This packet starts with byte 379642 and contains 200 bytes of data." The receiver can detect missing or incorrectly sequenced packets. TCP acknowledges data that has been received and retransmits data that has been lost. The TCP design means that error recovery is done end-to-end between the Client and Server machine. There is no formal standard for tracking problems in the middle of the network, though each network has adopted some ad hoc tools.

Need to Know

There are three levels of TCP/IP knowledge. Those who administer a regional or national network must design a system of long distance phone lines, dedicated routing devices, and very large configuration files. They must know the IP numbers and physical locations of thousands of subscriber networks. They must also have a formal network monitor strategy to detect problems and respond quickly.

Each large company or university that subscribes to the Internet must have an intermediate level of network organization and expertise. A half dozen routers might be configured to connect several dozen departmental LANs in several buildings. All traffic outside the organization would typically be routed to a single connection to a regional network provider.

However, the end user can install TCP/IP on a personal computer without any knowledge of either the corporate or regional network. Three pieces of information are required:

The IP address assigned to this personal computer
The part of the IP address (the subnet mask) that distinguishes other machines on the same LAN (messages can be sent to them directly) from machines in other departments or elsewhere in the world (which are sent to a router machine)
The IP address of the router machine that connects this LAN to the rest of the world.

In the case of the PCLT server, the IP address is 130.132.59.234. Since the first three bytes designate this department, a "subnet mask" is defined as 255.255.255.0 (255 is the largest byte value and represents the number with all bits turned on). It is a Yale convention (which we recommend to everyone) that the router for each department have station number 1 within the department network. Thus the PCLT router is 130.132.59.1. Thus the PCLT server is configured with the values:

My IP address: 130.132.59.234
Subnet mask: 255.255.255.0
Default router: 130.132.59.1

The subnet mask tells the server that any other machine with an IP address beginning 130.132.59.* is on the same department LAN, so messages are sent to it directly. Any IP address beginning with a different value is accessed indirectly by sending the message through the router at 130.132.59.1 (which is on the departmental LAN).

Additional information is available in self-study courses from SRA (1-800-SRA-1277)

TCP/IP [34610]

Copyright 1995 PCLT -- Introduction to TCP/IP -- H. Gilbert
This document generated by SpHyDir another fine product of PC Lube and Tune.

2008-06-15T21:46:00.002+07:00

OSI 7 Layers Reference Model For Network Communication

Open Systems Interconnection (OSI) model is a reference model developed by ISO (International Organization for Standardization) in 1984, as a conceptual framework of standards for communication in the network across different equipment and applications by different vendors. It is now considered the primary architectural model for inter-computing and internetworking communications. Most of the network communication protocols used today have a structure based on the OSI model. The OSI model defines the communications process into 7 layers, which divides the tasks involved with moving information between networked computers into seven smaller, more manageable task groups. A task or group of tasks is then assigned to each of the seven OSI layers. Each layer is reasonably self-contained so that the tasks assigned to each layer can be implemented independently. This enables the solutions offered by one layer to be updated without adversely affecting the other layers.

The OSI 7 layers model has clear characteristics. Layers 7 through 4 deal with end to end communications between data source and destinations. Layers 3 to 1 deal with communications between network devices.

On the other hand, the seven layers of the OSI model can be divided into two groups: upper layers (layers 7, 6 & 5) and lower layers (layers 4, 3, 2, 1). The upper layers of the OSI model deal with application issues and generally are implemented only in software. The highest layer, the application layer, is closest to the end user. The lower layers of the OSI model handle data transport issues. The physical layer and the data link layer are implemented in hardware and software. The lowest layer, the physical layer, is closest to the physical network medium (the wires, for example) and is responsible for placing data on the medium.

The specific description for each layer is as follows:

Layer 7:Application Layer

Defines interface to user processes for communication and data transfer in network
Provides standardized services such as virtual terminal, file and job transfer and operations

Layer 6:Presentation Layer

Masks the differences of data formats between dissimilar systems

Specifies architecture-independent data transfer format

Encodes and decodes data; Encrypts and decrypts data; Compresses and decompresses data

Layer 5:Session Layer
Manages user sessions and dialogues

Controls establishment and termination of logic links between users

Reports upper layer errors

Layer 4:Transport Layer

Manages end-to-end message delivery in network

Provides reliable and sequential packet delivery through error recovery and flow control
mechanisms

Provides connectionless oriented packet delivery

Layer 3:Network Layer

Determines how data are transferred between network devices

Routes packets according to unique network device addresses

Provides flow and congestion control to prevent network resource depletion

Layer 2:Data Link Layer

Defines procedures for operating the communication links

Frames packets

Detects and corrects packets transmit errors

Layer 1:Physical Layer

Defines physical means of sending data over network devices

Interfaces between network medium and devices

Defines optical, electrical and mechanical characteristics

There are other network architecture models, such as IBM SNA (Systems Network Architecture) model . Those models will be discussed in separate documents.

The OSI 7 layer model is defined by ISO in document 7498 and ITU X.200, X.207, X.210, X.211, X.212, X.213, X.214, X.215, X.217 and X.800. The protocols defined by ISO based on the OSI 7 layer mode are as follows:

Application
ACSE: Association Control Service Element

CMIP: Common Management Information Protocol

CMIS: Common Management Information Service

CMOT: CMIP over TCP/IP

FTAM: File Transfer Access and Management

ROSE: Remote Operation Service Element

RTSE: Reliable Transfer Service Element Protocol

VTP: ISO Virtual Terminal Protocol

X.400: Message Handling Service (ISO email transmission service) Protocols

X.500: Directory Access Service Protocol (DAP)

Presentation Layer
ISO-PP: OSI Presentation Layer Protocol

ASN.1: Abstract Syntax Notation One

Session Layer
ISO-SP: OSI Session Layer Protocol

Transport Layer
ISO-TP: OSI Transport Protocols: TP0, TP1, TP2, TP3, TP4

Network Layer
ISO-IP: CLNP: Connectionless Network Protocol

CONP: Connection-Oriented Network Protocol

ES-IS: End System to Intermediate System Routing Exchange protocol

IDRP: Inter-Domain Routing Protocol

IS-IS: Intermediate System to Intermediate System

Data Link
HDLC: High Level Data Link Control protocol

LAPB: Link Access Procedure Balanced for X.25

Reference: http://www.doc.ua.pt/arch/itu/rec/product/X.htm :

Wireless LAN

2008-06-15T21:01:00.005+07:00

From Wikipedia, the free encyclopedia

A wireless LAN or WLAN is a wireless local area network, which is the linking of two or more computers or devices without using wires. WLAN utilizes spread-spectrum or OFDM modulation technology based on radio waves to enable communication between devices in a limited area, also known as the basic service set. This gives users the mobility to move around within a broad coverage area and still be connected to the network.

For the home user, wireless has become popular due to ease of installation, and location freedom with the gaining popularity of laptops. Public businesses such as coffee shops or malls have begun to offer wireless access to their customers; some are even provided as a free service. Large wireless network projects are being put up in many major cities. Google is even providing a free service to Mountain View, California [1] and has entered a bid to do the same for San Francisco.[2] New York City has also begun a pilot program to cover all five boroughs of the city with wireless Internet access.

Contents

1 History
2 Benefits
3 Disadvantages
4 Architecture
4.1 Stations
4.2 Basic service set
4.3 Extended service set
4.4 Distribution system
5 Types of wireless LANs
5.1 Peer-to-peer
5.2 Bridge
5.3 Wireless distribution system
6 Roaming
7 See also
8 References
9 External links

History
In 1970 University of Hawaii, under the leadership of Norman Abramson, developed the world’s first computer communication network using low-cost ham-like radios, named ALOHAnet. The bi-directional star topology of the system included seven computers deployed over four islands to communicate with the central computer on the Oahu Island without using phone lines.

"In 1979, F.R. Gfeller and U. Bapst published a paper in the IEEE Proceedings reporting an experimental wireless local area network using diffused infrared communications. Shortly thereafter, in 1980, P. Ferrert reported on an experimental application of a single code spread spectrum radio for wireless terminal communications in the IEEE National Telecommunications Conference. In 1984, a comparison between Infrared and CDMA spread spectrum communications for wireless office information networks was published by Kaveh Pahlavan in IEEE Computer Networking Symposium which appeared later in the IEEE Communication Society Magazine. In May 1985, the efforts of Marcus led the FCC to announce experimental ISM bands for commercial application of spread spectrum technology. Later on, M. Kavehrad reported on an experimental wireless PBX system using code division multiple access. These efforts prompted significant industrial activities in the development of a new generation of wireless local area networks and it updated several old discussions in the portable and mobile radio industry.

The first generation of wireless data modems was developed in the early 1980's by amateur radio operators. They added a voice band data communication modem, with data rates below 9600 bit/s, to an existing short distance radio system, typically in the two meter amateur band. The second generation of wireless modems was developed immediately after the FCC announcement in the experimental bands for non-military use of the spread spectrum technology. These modems provided data rates on the order of hundreds of kbit/s. The third generation of wireless modem [then] aimed at compatibility with the existing LANs with data rates on the order of Mbit/s. Several companies [developed] the third generation products with data rates above 1 Mbit/s and a couple of products [had] already been announced [by the time of the first IEEE Workshop on Wireless LANs].

"The first of the IEEE Workshops on Wireless LAN was held in 1991. At that time early wireless LAN products had just appeared in the market and the IEEE 802.11 committee had just started its activities to develop a standard for wireless LANs. The focus of that first workshop was evaluation of the alternative technologies. [By 1996], the technology [was] relatively mature, a variety of applications [had] been identified and addressed and technologies that enable these applications [were] well understood. Chip sets aimed at wireless LAN implementations and applications, a key enabling technology for rapid market growth, [were] emerging in the market. Wireless LANs [were being] used in hospitals, stock exchanges, and other in building and campus settings for nomadic access, point-to-point LAN bridges, ad-hoc networking, and even larger applications through internetworking. The IEEE 802.11 standard and variants and alternatives, such as the wireless LAN interoperability forum and the European HiperLAN specification had made rapid progress, and the unlicensed PCS [ Unlicensed Personal Communications Services and the proposed SUPERNet, later on renamed as U-NII, bands also presented new opportunities.

On July 21, 1999, AirPort debuted at the Macworld Expo in New York City with Steve Jobs picking up an iBook supposedly to give the cameraman a better shot as he surfed the Web. Applause quickly built as people realized there were no wires. This was the first time Wireless LAN became publicly available at consumer pricing and easily available for home use. Before the release of the Airport, Wireless LAN was too expensive for consumer use and used exclusively in large corporate settings.
Originally WLAN hardware was so expensive that it was only used as an alternative to cabled LAN in places where cabling was difficult or impossible. Early development included industry-specific solutions and proprietary protocols, but at the end of the 1990s these were replaced by standards, primarily the various versions of IEEE 802.11 (Wi-Fi). An alternative ATM-like 5 GHz standardized technology, HiperLAN/2, has so far not succeeded in the market, and with the release of the faster 54 Mbit/s 802.11a (5 GHz) and 802.11g (2.4 GHz) standards, almost certainly never will.

In November 2006, the Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) won a legal battle in the US federal court of Texas against Buffalo Technology which found the US manufacturer had failed to pay royalties on a US WLAN patent CSIRO had filed in 1996. CSIRO are currently engaged in legal cases with computer companies including Microsoft, Intel, Dell, Hewlett-Packard and Netgear which argue that the patent is invalid and should negate any royalties paid to CSIRO for WLAN-based products.

Benefits

The popularity of wireless LANs is a testament primarily to their convenience, cost efficiency, and ease of integration with other networks and network components. The majority of computers sold to consumers today come pre-equipped with all necessary wireless LAN technology.
The benefits of wireless LANs include:

Convenience: The wireless nature of such networks allows users to access network resources from nearly any convenient location within their primary networking environment (home or office). With the increasing saturation of laptop-style computers, this is particularly relevant.

Mobility: With the emergence of public wireless networks, users can access the internet even outside their normal work environment. Most chain coffee shops, for example, offer their customers a wireless connection to the internet at little or no cost.

Productivity: Users connected to a wireless network can maintain a nearly constant affiliation with their desired network as they move from place to place. For a business, this implies that an employee can potentially be more productive as his or her work can be accomplished from any convenient location.

Deployment: Initial setup of an infrastructure-based wireless network requires little more than a single access point. Wired networks, on the other hand, have the additional cost and complexity of actual physical cables being run to numerous locations (which can even be impossible for hard-to-reach locations within a building).

Expandability: Wireless networks can serve a suddenly-increased number of clients with the existing equipment. In a wired network, additional clients would require additional wiring.

Cost: Wireless networking hardware is at worst a modest increase from wired counterparts. This potentially increased cost is almost always more than outweighed by the savings in cost and labor associated to running physical cables.

Disadvantages

Wireless LAN technology, while replete with the conveniences and advantages described above, has its share of downfalls. For a given networking situation, wireless LANs may not be desirable for a number of reasons. Most of these have to do with the inherent limitations of the technology.

Security: Wireless LAN transceivers are designed to serve computers throughout a structure with uninterrupted service using radio frequencies. Because of space and cost, the antennas typically present on wireless networking cards in the end computers are generally relatively poor. In order to properly receive signals using such limited antennas throughout even a modest area, the wireless LAN transceiver utilizes a fairly considerable amount of power. What this means is that not only can the wireless packets be intercepted by a nearby adversary's poorly-equipped computer, but more importantly, a user willing to spend a small amount of money on a good quality antenna can pick up packets at a remarkable distance; perhaps hundreds of times the radius as the typical user. In fact, there are even computer users dedicated to locating and sometimes even cracking into wireless networks, known as wardrivers. On a wired network, any adversary would first have to overcome the physical limitation of tapping into the actual wires, but this is not an issue with wireless packets. To combat this consideration, wireless networks users usually choose to utilize various encryption technologies available such as Wi-Fi Protected Access (WPA). Some of the older encryption methods, such as WEP are known to have weaknesses that a dedicated adversary can compromise. (See main article: Wireless security.)

Range: The typical range of a common 802.11g network with standard equipment is on the order of tens of metres. While sufficient for a typical home, it will be insufficient in a larger structure. To obtain additional range, repeaters or additional access points will have to be purchased. Costs for these items can add up quickly. Other technologies are in the development phase, however, which feature increased range, hoping to render this disadvantage irrelevant. (See WiMAX)
Reliability: Like any radio frequency transmission, wireless networking signals are subject to a wide variety of interference, as well as complex propagation effects (such as multipath, or especially in this case Rician fading) that are beyond the control of the network administrator. One of the most insidious problems that can affect the stability and reliability of a wireless LAN is the microwave oven.[7] In the case of typical networks, modulation is achieved by complicated forms of phase-shift keying (PSK) or quadrature amplitude modulation (QAM), making interference and propagation effects all the more disturbing. As a result, important network resources such as servers are rarely connected wirelessly.
Speed: The speed on most wireless networks (typically 1-108 Mbit/s) is reasonably slow compared to the slowest common wired networks (100 Mbit/s up to several Gbit/s). There are also performance issues caused by TCP and its built-in congestion avoidance. For most users, however, this observation is irrelevant since the speed bottleneck is not in the wireless routing but rather in the outside network connectivity itself. For example, the maximum ADSL throughput (usually 8 Mbit/s or less) offered by telecommunications companies to general-purpose customers is already far slower than the slowest wireless network to which it is typically connected. That is to say, in most environments, a wireless network running at its slowest speed is still faster than the internet connection serving it in the first place. However, in specialized environments, higher throughput through a wired network might be necessary. Newer standards such as 802.11n are addressing this limitation and will support peak throughputs in the range of 100-200 Mbit/s.

Wireless LANs present a host of issues for network managers. Unauthorized access points, broadcasted SSIDs, unknown stations, and spoofed MAC addresses are just a few of the problems addressed in WLAN troubleshooting. Most network analysis vendors, such as Network Instruments, Network General, and Fluke, offer WLAN troubleshooting tools or functionalities as part of their product line.

Architecture

Stations
All components that can connect into a wireless medium in a network are referred to as stations.
All stations are equipped with wireless network interface cards (WNICs).
Wireless stations fall into one of two categories: access points, and clients.
Access points (APs) are base stations for the wireless network. They transmit and receive radio frequencies for wireless enabled devices to communicate with.
Wireless clients can be mobile devices such as laptops, personal digital assistants, IP phones, or fixed devices such as desktops and workstations that are equipped with a wireless network interface.

Basic service set
The basic service set (BSS) is a set of all stations that can communicate with each other.
There are two types of BSS: Independent BSS ( also referred to as IBSS ), and infrastructure BSS.
Every BSS has an identification (ID) called the BSSID, which is the MAC address of the access point servicing the BSS.
An independent BSS (IBSS) is an ad-hoc network that contains no access points, which means they can not connect to any other basic service set.
An infrastructure BSS can communicate with other stations not in the same basic service set by communicating through access points.

Extended service set
An extended service set (ESS) is a set of connected BSSes. Access points in an ESS are connected by a distribution system. Each ESS has an ID called the SSID which is a 32-byte (maximum) character string. For example, "linksys" is the default SSID for Linksys routers.

Distribution system
A distribution system connects access points in an extended service setup. The concept of a DS can be to increase network coverage through roaming between cell's.

Types of wireless LANs

Peer-to-peer

Peer-to-Peer or ad-hoc wireless LAN
An ad-hoc network is a network where stations communicate only peer to peer (P2P). There is no base and no one gives permission to talk. This is accomplished using the Independent Basic Service Set (IBSS).

A peer-to-peer (P2P) allows wireless devices to directly communicate with each other. Wireless devices within range of each other can discover and communicate directly without involving central access points. This method is typically used by two computers so that they can connect to each other to form a network.
If a signal strength meter is used in this situation, it may not read the strength accurately and can be misleading, because it registers the strength of the strongest signal, which may be the closest computer.

802.11 specs define the physical layer (PHY) and MAC (Media Access Control) layers. However, unlike most other IEEE specs, 802.11 includes three alternative PHY standards: diffuse infrared operating at 1 Mbit/s in; frequency-hopping spread spectrum operating at 1 Mbit/s or 2 Mbit/s; and direct-sequence spread spectrum operating at 1 Mbit/s or 2 Mbit/s. A single 802.11 MAC standard is based on CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). The 802.11 specification includes provisions designed to minimize collisions. Because two mobile units may both be in range of a common access point, but not in range of each other. The 802.11 has two basic modes of operation: Ad hoc mode enables peer-to-peer transmission between mobile units. Infrastructure mode in which mobile units communicate through an access point that serves as a bridge to a wired network infrastructure is the more common wireless LAN application the one being covered. Since wireless communication uses a more open medium for communication in comparison to wired LANs, the 802.11 designers also included a shared-key encryption mechanism, called wired equivalent privacy (WEP), or Wi-Fi Protected Access, (WPA, WPA2) to secure wireless computer networks.

Bridge
A bridge can be used to connect networks, typically of different types. A wireless Ethernet bridge allows the connection of devices on a wired Ethernet network to a wireless network. The bridge acts as the connection point to the Wireless LAN.

Wireless distribution system
Main article: Wireless Distribution System
When it is difficult to connect all of the access points in a network by wires, it is also possible to put up access points as repeaters.

Roaming
There are 2 definitions for roaming in WLAN:
Internal Roaming (1): The Mobile Station (MS) moves from one access point (AP) to another AP within a home network because the signal strength is too weak. An authentication server (RADIUS) assumes the re-authentication of MS via 802.1x (e.g. with PEAP). The billing of QoS is in the home network.
External Roaming (2): The MS(client) moves into a WLAN of an another Wireless Service Provider (WSP) and takes their services (Hotspot). The user can independently of his home network use another foreign network, if this is open for visitors. There must be special authentication and billing systems for mobile services in a foreign network

LAN TOPOLOGY

2008-06-15T20:24:00.015+07:00

There are four basic types of LAN topology.

STAR
RING
BUS
TREE

STAR NETWORK

In the star LAN topology, each station is directly connected to a common central node. Typically, each station attaches to a central node, referred to as the star coupler, via two point-to-point links, one for transmission and one for reception. In general, there are two alternatives for the operation of the central node. One approach is for the central node to operate in a broadcast fashion. A transmission of a frame from one station to the node is retransmitted on all of the outgoing links. In this case, although the arrangement is physically a star, it is logically a bus; a transmission from any station is received by all other stations, and only one station at a time may successfully transmit. Another approach is for the central node to act as a frame switching device. An incoming frame is buffered in the node and then retransmitted on an outgoing link to the destination station.

RING TOPOLOGY

In the ring topology, the network consists of a set of repeaters joined by point-topoint links in a closed loop. The repeater is a comparatively simple device, capable of receiving data on one link and transmitting them, bit by bit, on the other link as fast as they are received, with no buffering at the repeater. The links are unidirectional; that is, data are transmitted in one direction only and all are oriented in the same way. Thus, data circulate around the ring in one direction (clockwise or counterclockwise).Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. As with the bus and tree, data are transmitted in frames. As a frame circulates past all the other stations, the destination station recognizes its address and copies the frame into a local buffer as it goes by. The frame continues to circulate until it returns to the source station, where it is removed. Because multiple stations share the ring, medium access control is needed to determine at what time each station may insert frames.

BUS TOPOLOGY

For the bus, all stations attach, through appropriate hardware interfacing known as a tap, directly to a linear transmission medium, or bus. Full-duplex operation between the station and the tap allows data to be transmitted onto the bus and received from the bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. At each end of the bus is a terminator, which absorbs any signal, removing it from the bus.

TREE TOPOLOGY

The tree topology is a generalization of the bus topology. The transmission medium is a branching cable with no closed loops. The tree layout begins at a point known as the headend, where one or more cables start, and each of these may have branches. The branches in turn may have additional branches to allow quite complex layouts. Again, a transmission from any station propagates throughout the medium and can be received by all other stations. Two problems present themselves in this arrangement. First, because a transmission from any one station can be received by all other stations, there needs to be some way of indicating for whom the transmission is intended. Second, a mechanism is needed to regulate transmission.

INTRODUCTION TO LAN

This web site is about LAN AND LAN TOPOLOGY.

AN EXAMPLE OF LOCAL AREA NET WORK
LANs are the high speed, low-error data netorks that span a relatively small geographic area. they connect workstations,peripherals,terminals, and other devices in a single building or other geographically limited area.

The LANs are distinguished from other types of data networks in that they are optimized for a moderate size geographic area such as a single office building, a warehouse, or a campus. The IEEE 802 LAN is a shared medium peer-to-peer communications network that broadcasts information for all stations to receive. As a consequence, it does not inherently provide privacy. The LAN enables stations to communicate directly using a common physical medium on a point-to-point basis without any intermediate switching node being required. There is always need for an access sublayer in order to arbitrate the access to the shared medium. The network is generally owned, used, and operated by a single organization. This is in contrast to Wide Area Networks (WANs) that interconnect communication facilities in different parts of a country or are used as a public utility. These LANs are also different from networks, such as backplane buses, that are optimized for the interconnection of devices on a desk top or components within a single piece of equipment

LAN COMPONENTS
The components of LAN are as following:

WORKSTATIONS
NETWORKING MEDIA
NIC CARDS

WORKSTATION

UNSHIELDED TWISTED-PAIR CABLE

COAXIAL CABLE

Protocol (computing)

2008-06-15T20:16:00.002+07:00

From Wikipedia, the free encyclopedia
In computing, a protocol is a convention or standard that controls or enables the connection, communication, and data transfer between two computing endpoints. In its simplest form, a protocol can be defined as the rules governing the syntax, semantics, and synchronization of communication. Protocols may be implemented by hardware, software, or a combination of the two. At the lowest level, a protocol defines the behavior of a hardware connection.

Meaning: Set of rules.
Contents
1 Typical properties
2 Importance
3 Common protocols
4 Protocol testing
5 See also

Typical properties
It is difficult to generalize about protocols because they vary so greatly in purpose and sophistication. Most protocols specify one or more of the following properties:
Detection of the underlying physical connection (wired or wireless), or the existence of the other endpoint or node

Handshaking
Negotiation of various connection characteristics
How to start and end a message
How to format a message
What to do with corrupted or improperly formatted messages (error correction)
How to detect unexpected loss of the connection, and what to do next
Termination of the session or connection.

Importance
The widespread use and expansion of communications protocols is both a prerequisite for the Internet, and a major contributor to its power and success. The pair of Internet Protocol (or IP) and Transmission Control Protocol (or TCP) are the most important of these, and the term TCP/IP refers to a collection (or protocol suite) of its most used protocols. Most of the Internet's communication protocols are described in the RFC documents of the Internet Engineering Task Force (or IETF).

The protocols in human communication are separate rules about appearance, speaking, listening and understanding. All these rules, also called protocols of conversation, represent different layers of communication. They work together to help people successfully communicate. The need for protocols also applies to network devices. Computers have no way of learning protocols, so network engineers have written rules for communication that must be strictly followed for successful host-to-host communication. These rules apply to different layers of sophistication such as which physical connections to use, how hosts listen, how to interrupt, how to say good-bye,in short how to communicate, what language to use and many others. These rules, or protocols, that work together to ensure successful communication are groups into what is known as a protocol suite.
Object-oriented programming has extended the use of the term to include the programming protocols available for connections and communication between objects.
Generally, only the simplest protocols are used alone. Most protocols, especially in the context of communications or networking, are layered together into protocol stacks where the various tasks listed above are divided among different protocols in the stack.
Whereas the protocol stack denotes a specific combination of protocols that work together, a reference model is a software architecture that lists each layer and the services each should offer. The classic seven-layer reference model is the OSI model, which is used for conceptualizing protocol stacks and peer entities. This reference model also provides an opportunity to teach more general software engineering concepts like hiding, modularity, and delegation of tasks. This model has endured in spite of the demise of many of its protocols (and protocol stacks) originally sanctioned by the ISO. The OSI model is not the only reference model however.

Common protocols
IP (Internet Protocol)
UDP (User Datagram Protocol)
TCP (Transmission Control Protocol)
DHCP (Dynamic Host Configuration Protocol)
HTTP (Hypertext Transfer Protocol)
FTP (File Transfer Protocol)
Telnet (Telnet Remote Protocol)
SSH (Secure Shell Remote Protocol)
POP3 (Post Office Protocol 3)
SMTP (Simple Mail Transfer Protocol)
IMAP (Internet Message Access Protocol)

Protocol testing
In general, protocol testers work by capturing the information exchanged between a Device Under Test (DUT) and a reference device known to operate properly. In the example of a manufacturer producing a new keyboard for a personal computer, the Device Under Test would be the keyboard and the reference device, the PC. The information exchanged between the two devices is governed by rules set out in a technical specification called a "communication protocol". Both the nature of the communication and the actual data exchanged are defined by the specification. Since communication protocols are state-dependent (what should happen next depends on what previously happened), specifications are complex and the documents describing them can be hundreds of pages.

The captured information is decoded from raw digital form into a human-readable format that permits users of the protocol tester to easily review the exchanged information. Protocol testers vary in their abilities to display data in multiple views, automatically detect errors, determine the root causes of errors, generate timing diagrams, etc.

Some protocol testers can also generate traffic and thus act as the reference device. Such testers generate protocol-correct traffic for functional testing, and may also have the ability to deliberately introduce errors to test for the DUT's ability to deal with error conditions.

Protocol testing is an essential step towards commercialization of standards-based products. It help ensure that products from different manufacturers will operate together properly ("interoperate") and so satisfy customer expectations. This type of testing is of particular importance for new emerging communication technologies.

See also
Internet protocol suite
Communications protocol
List of network protocols
Application programming interface
Calling convention
Retrieved from "http://en.wikipedia.org/wiki/Protocol_%28computing%29"

protocol (network)

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By Bradley Mitchell, About.com

Definition: A network protocol defines rules and conventions for communication between network devices. Protocols for computer networking all generally use packet switching techniques to send and receive messages in the form of packets.

Network protocols include mechanisms for devices to identify and make connections with each other, as well as formatting rules that specify how data is packaged into messages sent and received. Some protocols also support message acknowledgement and data compression designed for reliable and/or high-performance network communication. Hundreds of different computer network protocols have been developed each designed for specific purposes and environments.

Internet Protocols
The Internet Protocol family contains a set of related (and among the most widely used network protocols. Besides Internet Protocol (IP) itself, higher-level protocols like TCP, UDP, HTTP, and FTP all integrate with IP to provide additional capabilities. Similarly, lower-level Internet Protocols like ARP and ICMP also co-exist with IP. These higher level protocols interact more closely with applications like Web browsers while lower-level protocols interact with network adapters and other computer hardware.

Routing Protocols
Routing protocols are special-purpose protocols designed specifically for use by network routers on the Internet. Common routing protocols include EIGRP, OSPF and BGP.

How Network Protocols Are Implemented
Modern operating systems like Microsoft Windows contain built-in services or daemons that implement support for some network protocols. Applications like Web browsers contain software libraries that support the high level protocols necessary for that application to function. For some lower level TCP/IP and routing protocols, support is implemented in directly hardware (silicon chipsets) for improved performance.
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Conclusion

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6. CONCLUSION

While the age-old concept of the network is foundational in virtually all areas of society, Computer Networks and Protocols have forever changed the way humans will work, play, and communicate. Forging powerfully into areas of our lives that no one had expected, digital networking is further empowering us for the future. New protocols and standards will emerge, new applications will be conceived, and our lives will be further changed and enhanced. While the new will only be better, the majority of digital networking's current technologies are not cutting-edge, but rather are protocols and standards conceived at the dawn of the digital networking age that have stood solid for over thirty years.

The Internet: The Ultimate Network

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5. THE INTERNET: THE ULTIMATE NETWORK

The Internet, currently an essential part of a fast-growing number of areas of life, has truly changed the world. Purely a network and entirely based upon OSI Reference Model compliant protocols, the Internet takes everything discussed here and scales it unimaginably. The success of the internet truly shows the flexibility and power of concepts that, when compared in this context, seem relatively simple.

Due to its scale and utilization of many different technologies and concepts, the design of the internet can not be credited to one particular individual. Rather, over time many researchers, students, corporate organizations, and individuals have contributed many technologies and ideas to form what the Internet is today. While the modern day typical internet user thinks of the internet as being web pages and email, in reality the name 'Internet' is referring simply the system on top of which web browsing, email, and innumerable other communication systems operate. The Internet, simply put, is a global network of networks. It is unquestionably the largest switched/routed IP network in existence.

The Internet Protocol Suite:

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4. THE INTERNET PROTOCOL SUITE: A LESSON IN PROTOCOL STACKS

A Protocol Stack is a group of protocols that follows the specification of several consecutive OSI Layers. This section will provide an example of a Protocol Stack/Suite by dissecting the Internet Protocol Suite.

4.1 INTRODUCTION TO THE INTERNET PROTOCOL SUITE

The Internet Protocol Suite is a stack of protocols based on the OSI Reference Model. Undeniably the single most used Protocol Stack in the world, the IP Suite is the primary power behind the internet and a large number of other networks of all sizes. This suite is known as the TCP/IP suite or the IP Suite, despite the fact that it is actually a suite of specifications and consists of more than just the TCP and IP protocols. To make things easier to understand, the TCP/IP suite is often explained using just four layers, each of which represents multiple OSI layers.

4.2 THE INTERNET PROTOCOL SUITE LINK LAYER

While not technically a part of the Internet Protocol Suite, the IP Suite relies on a link layer, just as any other protocol stack would. Without the Link Layer, which represents OSI Layers One and Two, the higher protocols defined in the TCP/IP stack would not function.
An interesting advanced topic that can be considered here is the concept of a Virtual Private Network (VPN) or network "tunnel". A network tunnel links two remote local area networks as if they were one local area network. This operates by running a VPN stack with a TCP/IP stack on top. While this concept may seem complex, the same principles discussed earlier in this document in relation to stacking apply not only to protocols, but to stacks of protocols. While theoretically this concept could extend without limits, it never really does due to protocol overhead (the space consumed by packet headers) and the fact that no widespread practical use has ever existed for more than two or three nested tunnels.

4.3 THE INTERNET PROTOCOL SUITE INTERNETWORK LAYER

Also known as the Internet Layer, due to its almost exclusive use on the medium, this is the level at which packets are routed and switched on networks. The Internet Protocol (IP, not to be confused with the IP Suite) is responsible for getting this job done. As shown when being used for an example on routing in Diagram 3, IP addresses are determined and assigned to each node by IP. IP is an OSI Layer 3 protocol.

4.4 THE INTERNET PROTOCOL SUITE TRANSPORT LAYER

The Internet Protocol Suite's Transport Layer is where the TCP/IP suite shows its broad diversity and capability. Supporting multiple varied mainstream protocols, the IP Suite's Transport Layer provides many options for the protocol and associated feature-set that a node's applications may use. IP Suite Transport Layer protocols fall under the specification of OSI Layers Four and Five. Here are some of the most common IP Suite Transport Layer protocols.

TCP

TCP is a reliable, connection oriented protocol, and is possibly the most commonly used IP Suite Transport Layer protocol. Its advantages are that it is reliable, meaning it will attempt to re-send packets that fail to reach their destination with the same integrity with which they left. In order assist in preventing this issue, TCP attempts to monitor the current load and free capacity based on the action of other TCP network traffic and will throttle its packet sending rate to prevent network packet overload/collision. In addition, TCP will attempt to send packets in roughly the order they originally were intended. TCP performs best when used with an application that does not require timely, ordered information, but does require the information be of good integrity. TCP is classified as an OSI Layer 4 (Transport Layer) protocol.

UDP

Often viewed as being similar to TCP, UDP begins to differ in that it is an unreliable type protocol. This does not mean that it serves its purpose poorly, but rather that UDP does not verify that its packets have reached the destination node successfully, and will not put future packets on hold to retransmit current failed ones. This means that UDP is typically utilized in applications where the integrity of transmitted information is not particularly required, but timely delivery is. UDP is useful in such applications as multimedia streaming because it does not stop to resend bad packets, thus preventing pauses in the media stream. UDP is classified as an OSI Layer 4 (Transport Layer) protocol.

RTP
RTP is a Session Layer (OSI Layer 5) protocol that lies on top of UDP (an OSI Layer 4 protocol). RTP is specifically designed to deliver streaming audio and video content on time and in order. Utilizing UDP for its unreliable time-conscious transmission methods, RTP ensures that packets reach the end node's application both in a timely manner and in the originally intended order.

4.5 THE INTERNET PROTOCOL SUITE APPLICATION LAYER

The IP Suite's Application Layer is where things the common user interacts with come into play. Representing the OSI Reference Model's Layers Six and Seven, the IP Suite has a large number of protocols commonly used on its highest layer.

THE UNIFORM RESOURCE LOCATOR CONCEPT

In order to allow the IP Suite the flexibility to operate using a variety of Transport and Application layer protocols, the need for a uniform way to reference these protocol's resources arises. The IP Suite uses a system known as the Uniform Resource Locator (URL). A URL, as shown in Diagram 5, commonly consists of three parts, but various protocols may have an expanded syntax13 to reflect expanded capability.

The first segment of a URL indicates the Application Layer protocol that will be used for this request. Common examples are http://, https://, and ftp://. The second segment of a URL is an IP address or Host Name14. This tells the IP protocol (OSI Layer 4) the IP (logical) address of the node where the requested resource is located. The third segment of the IP address, indicated in Diagram 5 by the position of the number '80', is the Port Number. The concept of Layer 4 ports is introduced in Section 2.4 of this document. The Port Number in a URL tells the IP protocol the remote port it should attempt to access. Diagram 5 is a URL telling the IP Suite that it should use the HTTP protocol to access the HTTP protocol operating on port 80 of the node located at 127.0.0.1.

HTTP

Possibly the most recognizable protocol yet discussed here, HTTP is the HyperText Transport Protocol. Following the specification of OSI Layer 7, the HTTP protocol is responsible for fetching, sending, and receiving files per the requests of the end user. HTTP is commonly used inside of computer programs called browsers15 to allow for the quick viewing of many filetypes and for ease of navigation among them.

The average person would likely recognize HTTP as being 'those four letters typed at the beginning of a web page address', and would be correct since websites operate primarily on the HTTP protocol. Thus a website's URL might look something like: http://NSGN.net. HTTP typically operates by default on TCP Port 80.

HTTPS

HTTPS operates identically in every way to HTTP except that it encrypts all packets it handles on-the-fly. HTTPS requires an encryption certificate to operate properly. A certificate is a digital document that only the two end users transferring information via HTTPS posses. The certificate contains the encryption/decryption key, thus the only end users able to make use of the information transmitted over the HTTPS connection are the two who hold certificates. HTTPS typically operates by default on TCP Port 443.

FTP

FTP is the File Transfer Protocol. It is a protocol used for the transferring of files between two nodes over a network. While FTP is far from being the only file transfer protocol designed to run on top of the IP Suite, it is one of the first and in many ways is unparalleled. The FTP protocol is commonly used through computer programs known as "FTP Clients". These software applications send and receive FTP commands and present the various information to the user. An FTP session, depending on the software application in use, may sometimes be initiated by a URL beginning with ftp:// . FTP typically operates by default on TCP Port 21.

SSH

SSH is the Secure SHell protocol. Used primarily on business or server computer operating systems, the SSH protocol allows a node to be remotely controlled or administrated. The SSH protocol typically operates by default on TCP Port 22.

4.6SUMMARY OF THE INTERNET PROTOCOL SUITE

Despite the popularity and large user base for the example IP suite protocols discussed here, numerous others exist. The specification is just as open as the OSI Reference Model, because in reality the IP Suite is simply a specification calling for OSI compliant protocols to communicate in the same method. The IP Suite is an excellent example of what is known as a de-facto standard.

13 Syntax; The rules for the construction of a command or statement.

14 Host Name; A unique name that identifies a computer or server on the Internet. In layman's terms, a name that points to an IP address. Host names are used primarily because they are easier for humans to comprehend and remember.

15 Browser; A computing program with a graphical user interface for displaying HTML files, used to navigate the World Wide Web : a Web browser.

Common Network Hardware

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MODERN NETWORK HARDWARE & INFRASTRUCTURE STANDARDS

Since the OSI Model is a reference rather than a standard, it leaves room for standards of many different designs and applications to be made in its image. Based upon OSI Layers One and Two, these standards fill the gaps that exist in both technology and geographical distance allowing digital communication to spreader farther, carry information faster, and cost less.

3.1 ETHERNET (IEEE 802.3)

Ethernet is used to link computers in both small residential and large commercial situations and is the most widely used Network Hardware Standard today. It often delivers internet access from other longer range hardware standards to multiple computers within a home or workplace. Ethernet equipment is relatively small, affordable, and can carry data at high speeds. The original specification called for speeds of 10mbps9, while newer technologies have brought that speed today to lie between 100mbps and 1000mbps. Ethernet is rarely used outside the of local area networks found inside of business or homes due to it's range limitations. Ethernet installations typically can not run for more than a few thousand feet. This leaves other network infrastructures to link computers over great distances. Ethernet most commonly forms what is known as the LAN, or Local Area Network. A LAN is a collection of computers in close proximity that are linked together to form a network. This network may then be linked to other LANs via network infrastructure standards with long distance capabilities.

Originally developed at Xerox's PARC10 facility, the project was predominately conceived and headed by a man named Robert Metcalfe. In 1976, Metcalfe and his team published a paper entitled Ethernet: DIstributed Packet Switching For Local Computer Networks which drew out conceptual specifications for the Ethernet standard. Though the standard defined in his paper was for 3mbps 8bit communication, Ethernet would soon evolve into its more modern-day form when Metcalfe left Xerox to form 3Com. In 1980, he encouraged major companies such as DEC, Intel, and Xerox to participate in a standard he called "DIX" (for DEC, Intel, Xerox). This standard defined Ethernet as having 10mbps speeds and would end up competing directly against the day's largest proprietary systems.

The Ethernet standard demonstrates its flexibility by supporting multiple transmission mediums. The original medium, known as 10BASE-2, utilized BNC11 type coaxial connections and coaxial cabling. This was the standard for many years, transmitting data at a rate of 10mbps. 10BASE-2, however, became increasingly cumbersome, requiring high maintenance. A complete circuit was required for proper operation, meaning that a single failed node or cable break on a large network cause a cease of proper operation. In the early 90s, the newer 10BASE-T standard emerged. 10BASE-T utilizes twisted pair cable, which is similar to copper phone lines but differs in that it carries four twisted pairs instead of one or two. Operating at speeds of either 10mbps, or later 100mbps (100BASE-T), this standard has become and remains the most widely used network standard in the world. In the late 90s, Gigabit Ethernet came into existence, allowing for transfer speeds of up to 1000mbps over the same twisted pair cabling. The Gigabit Ethernet standard is also capable of transmitting over optical cable, though this ability has not gained a following due to the existence of superior high end fiber optic network standards.

While there are numerous other standards operating under the IEEE 802.3 specification, most others are used in niche markets or private deployments for very large network backbones. Ethernet has been and will remain for years the most used standard for the transmission of digital information over short distances.

3.2 WI-FI (802.11x)

Wi-Fi is a standard developed to perform nearly the same role as Ethernet does in consumer settings, but without the wires. Taking to the air, Wi-Fi allows a node to lie anywhere within a 100 to 1000 foot range of a Wi-Fi enabled router and have a constant, secure connection to the Local Area Network. Wi-Fi originated with speeds of just 11mbps in the form of IEEE 802.11b, but today can achieve speeds between 54mbps and 108mbps.
In 1991, the original Wi-Fi standard was developed at AT&T by a man named Vic Hayes. It was initially designed to provide wireless communication for cashier systems in retail locations and operated at speeds of 1 or 2mbps. In the late 90s, the IEEE ratified the 802.11b specification, providing wireless ethernet-like connectivity for nodes at speed steps between 1 and 11mbps. The varied speeds allowed a node's hardware to switch to a lower transmission speed when further from the access point12 in order to maintain the connection over a longer distance.
In 2003 and 2004 IEEE 802.11g, a newer standard based on the Wi-Fi specification, emerged. 802.11g provides speeds between 11 and 54mbps while still maintaining backward compatibility with the older 802.11b standard and it's 1 to 11mbps speed range. This means 802.11g is able to offer superior speeds while still capable of reverting to lower speed, long range transmission rates when necessary.

In 2005, an even newer standard began to emerge known as 802.11n. While not officially ratified to date, so called "Pre-N" devices have begun to be sold in the consumer marketplace based upon the 802.11n standard that is still in the ratification process at the IEEE. The 802.11n specification calls for transmission speeds of 108 to 540mbps while still maintaining full support for the 802.11b/g standards speeds between 1 and 54mbps. While in the 802.11g standard the longer range, lower speed backward compatible 802.11b standard was utilized to increase the range and connection stability nodes received when further from the 'g' access point, 802.11n uses previous standards almost exclusively for compatibility with older equipment. This is due to the fact that 802.11n devices are able to communicate at 54 to 108mbps speeds at ranges greater than those offered by 802.11b when operating at 1 to 5mbps. 802.11n is not expected to begin to receive widespread adoption until late 2006 or 2007, both because it has not yet received IEEE certification and it has the current standard's enormous market saturation to attempt to replace.

The issue of range has greatly marred the performance of 802.11 equipment for years. This problem is obvious when it is considered that most 802.11b equipment actually only operates at 5 to 7mbps, and 802.11g equipment at 24 to 36mbps during real- world use. Fortunately, with the improvements brought by the 802.11n and future standards, this problem will begin to fade as the speeds achieved during everyday use close in on the technical maximum speeds offered by emerging standards.

3.3 BLUETOOTH (802.15.1)

Bluetooth, IEEE 802.15.1, is a short range wireless network standard originally developed by Ericsson Corporation. Designed to allow nodes to participate in a network using the lowest possible amount of power, Bluetooth supports three power/range levels: 1mW/10cm, 2.5mW/10m, and 100mW/100m. Bluetooth's current maximum transmission rate is 2.1mbps. While this seems very low compared to much older Wi-Fi standards such as 802.11b, Bluetooth is designed to fit a special section of the market, rather than to be a widespread, high-performance technology. Bluetooth is almost always used in a paired or "ad-hoc" type network. In an ad-hoc network, no router exists, but the nodes are simply responsible for negotiating communication among themselves automatically. A paired network is simply an ad-hoc network with only two nodes.

Common Bluetooth devices and applications include mobile phone headsets, PC-to-organizer/PDA synchronization, and other situations in which small devices need low power, short range communication capability. Bluetooth is a staple feature on most of today's newest and smallest portable information and communication devices.

9 mbps; Megabits per second. Equal to roughly one million bits per second.
10 PARC; The Xerox Palo Alto Research Center located in Palo Alto, California.
11 BNC; A type of RF connector used for terminating coaxial cable. The connector was named after its "Bayonet" locking mechanism and its two inventors "Neill" and "Concelman".
12 Access Point; Commonly the term for a wirelessly enabled network router or switch.

The OSI Reference Model

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2. THE OSI REFERENCE MODEL

The concept of how a modern day network operates can be understood by dissecting it into seven layers. This seven layer model is known as the OSI Reference Model and defines how the vast majority of the digital networks on earth function. OSI is the acronym for Open Systems Interconnection, which was an effort formed by the International Organization for Standardization in 1982 with the goal of producing a standard reference model for the hardware and software connection of digital equipment. The important concept to realize about the OSI Reference Model is that it does not define a network standard, but rather provides guidelines for the creation of network standards. The OSI has become so accurate a concept that almost all major network standards in use today conform entirely to it's seven layer model. Though seven layers may at first appear to make a network seem overly complex, the seven layer OSI Model has been proven over the past twenty years to be the most efficient and effective way to understand this extremely complex subject.

2.1 OSI LAYER 1: THE PHYSICAL LAYER

The first and foundational layer of a network is the Physical Layer. The Physical Layer is literally what it's name implies: the physical infrastructure of a network. This includes the cabling or other transmission medium and the network interface hardware placed inside of computers and other devices which enable them to connect to the transmission medium. The purpose of the Physical Layer is to take binary1 information from higher layers, translate it into a transmission signal or frequency, transmit the information across the transmission medium, receive this information at the destination, and finally translate it back into binary before passing it up to higher layers. Transmission signals or frequencies vary between network standards and can be as simple as pulses of electricity over copper wiring or as complex as flickers of light on optical lines or amplified radio frequency transmissions. The information that enters and exits the Physical Layer must be bits; either 0s or 1s in binary. The higher layers are responsible for providing the Physical Layer with binary information. Since nearly all information inside of a computer is already digital2, this is not difficult to achieve. The Physical Layer does not examine the binary information nor does it validate it or make changes to it. The Physical Layer is simply intended to transport the binary information between higher layers located at points A and B.

2.2 OSI LAYER 2: The DATA LINK LAYER

The second layer in the OSI Model is the Data Link Layer. The primary focus of the Data Link Layer is revealed in its common nickname, The Physical Address Layer. The only layer in the OSI Model that specifically addresses both hardware and software, the Data Link Layer receives information on its software side from higher layers, places this information inside of "frames", and finally gives this frame to the Physical Layer, Layer 1, for transmission as pure binary. A frame essentially takes the information passed down from a higher layer and surrounds it with Physical Addressing information. This information is important to the Data Link Layer on the receiving end of the transmission. When the frame, in binary form, arrives at the destination node3, it is passed from the transmission medium to the Data Link Layer (Layer 2) by the Physical Layer (Layer 1). The Data Link Layer on the receiving node then checks the frame surrounding the information received to see if it's Physical Address matches that of its own. If the Physical Address does not match, the frame and its encapsulated data is discarded. If the Physical Address is a match, then the information is removed from the frame and passed up to the next highest layer in the OSI Model. The Physical Address check is Obviously not of much use if there are only two nodes on a network, but suddenly becomes extremely valuable when three or more nodes exist. The Physical Addressing system allows multiple nodes to be on the same network medium, but retain the ability to address only a specific node with a transmission. On the simplest networks, all nodes receive every frame transmitted on the network, but discard frames not specifically addressed to them.
The Physical Address used in the Data Link Layer's Physical Addressing system is known as a MAC4 address and is embedded physically into the node's Network Interface Card during manufacturing. Every NIC's MAC address is unique in order to prevent addressing conflicts. It is this relationship that causes the Data Link Layer to be known as the only layer that addresses both hardware and software. This layer is where the information on the network makes the move from the physical infrastructure of the network into the software realm. The remainder of the OSI Reference Model's layers are entirely software.

2.3 OSI LAYER 3: THE NETWORK LAYER

OSI Layer 3 is known as the Network Layer. The first layer to deal entirely in software, the purpose of the Network Layer is to direct network traffic to a destination node who's Physical Address is not known. This is achieved through a system known as Logical Addressing. Logical Addresses are software addresses assigned to a node at Layer 3 of the OSI Model. Since these addresses are able to be defined by software rather than being random and permanent like Physical Addresses, Logical Addresses are able to be hierarchical. This allows extremely large networks to be possible. Up until this point, only small networks would be possible since all traffic was addressed to all nodes. This works fine until more than one person attempts to utilize the network at once, at which point a data "collision" occurs. While OSI Layer 4 protocols may attempt to compensate for this collision by retransmitting packets until they have reached the destination node without issue, this degrades network performance exponentially as the number of nodes on a network grows. The larger the network is the greater this issue becomes. OSI Layer 3 takes on this problem by its Logical Addressing system and a concept known as routing.
The Oxford American Dictionary defines routing as "Sending or directing along a specified course". Layer 3 routing on a network takes this foundational definition and puts it to use to enable millions of computers, rather than just a handful, to communicate at once without interference. This is achieved by having a smart device working at Layer 3 that handles network signals from each node directly rather than nodes just blindly repeating packets at Layer 1 until they happen to reach their destination. Such a device is known as a network router. A network router sits in the center of a network with all nodes having a direct link to it rather than being linked to each other. This strategic position allows the router to intercept and direct all traffic on the network. A routed network can be illustrated by a star formation, as shown in Diagram 1. On a routed network, Layer 3 packets are no longer broadcasted to all nodes, but rather received by the router and passed on only to the appropriate node. This is a valuable concept because it allows for the collision free-transport of packets across a network.

As well as being linked directly to all nodes in a local network, a router can be linked directly to other routers. This allows groups of nodes separated by distance to communicate with each other in a practical way. It would not be practical to have nodes separated by a great distance all connect to a single router. The amount of cabling required would be immense, and depending on the number of nodes involved, the router may not posses the required number of physical connections. Placing a router at each group of nodes and running a single line from router to router, however, is quite practical. Routers can be chained in a line, or as shown in Diagram 2, can be connected by a central router. This concept is virtually infinitely scalable and is very efficient.
When a node starts a transmission, the OSI Layer 3 protocol takes the information passed down from higher layers and encapsulates it with the logical address of the destination node in a unit called a packet. This packet, then passing through the remaining lower layer protocols, is transmitted over the network medium from the node to the router. This router reads the logical address that the packet contains and compares it to a list of physical addresses of nodes directly connected to it. If the packet's destination address matches an entry in this list, the packet is transmitted directly on the line that leads straight to the destination node. If the router does not know of a direct connection to the destination node, the packet is transmitted on a line leading directly to another router. This router then treats the packet much like the first router did upon receipt. The packet's logical address is checked for matches against the list of logical addresses belonging to nodes directly connected to the router. If the packet reaches a router with connections only to other routers, as shown in Diagram 2, the router uses the logical address' orderly numbering scheme to try and determine the closest router to the destination node and then transmits the packet to that router.

IP, undoubtedly the world's most used Layer 3 Protocol, provides an excellent example of how this system works. In IP, logical addresses look like four sets of up to three numbers.5 Diagram 3 shows an example of an IP address. IP addresses are orderly on four levels, from left to right. The first section of the IP address refers to a top level router, or a router that is at the highest level of this particular branch of the network. In Diagram 3, the first number is 66. Therefore all IP addresses between 66.0.0.1 and 66.255.255.255 are managed by this router. Only one router is required in a routed network, but more may exist. A router may have a maximum of 255 nodes, which may be either ordinary nodes or other routers. This effectively means that each branch of a network, a group of nodes that have the first set of numbers in their IP address in common, could theoretically have over sixteen million end nodes and still operate with near peak efficiency6.

As we can now see, the OSI Reference Model Layer 3 is one of the most complex, but most functionally important, parts of the modern day network. The Layer 3 protocol IP stands for Internet Protocol and is the protocol handling virtually all traffic on the internet today. The fashion in which Layer 3 protocols connect computers in a star-shaped, extensible network is much of the reason the internet is commonly called the "web".

2.4 OSI LAYER 4: THE TRANSPORT LAYER

OSI Layer 4 is known as the Transport Layer. Since we are now above Layer 3, all information transfered is assumed to be at the correct destination node and is being passed up to Layer 4. The Transport Layer is responsible for the reliability of the link between two end users and for dividing the data that is being transmitted by assigning port numbers to its Layer 4 packages, known as segments. Ports can be thought of as virtual destination mailboxes or outlets. When information reaches a Layer 4 protocol, the segment is examined to determine the destination port of the data it contains. Once the port is determined, just as all of the past layers have done, the wrapper is discarded and the payload data passed up to the next layer's protocol.
Ports allow more than one set of Layer 5-7 protocols to exist on a single node. This is important if the node has more than one purpose. Modern home computers utilize many ports during everyday use, because the modern computer user demands that a computer serve many purposes at once. Higher layer protocols that provide services such as email, web browsing, text chat, file transfer and more each operate on their own unique Layer 4 port, allowing all of these protocols to be operated at once without interference.
On the reliability front, Transport Layer protocols can be capable of running a checksum7 on the payload data they carry. This allows the protocol to determine the integrity of incoming payload data. If this data has been corrupted or its integrity compromised, the Layer 4 protocol will request the segment be retransmitted.

2.5 OSI LAYER 5: THE SESSION LAYER

While being an optional layer in most protocol packages today, OSI Layer 5, known as the Session Layer, still serves a purpose in the OSI Reference Model. The Session Layer draws the outline for protocols that manage the combination and synchronization of data from two separate higher layers. Layer 5 protocols are responsible for ensuring that the data is synced and consistent before transmitted. A good example situation is the streaming of live multimedia audio and video, where near perfect synchronization between video and audio is desired.

2.6 OSI LAYERS 6 & 7: THE PRESENTATION AND APPLICATION LAYERS

The sixth and seventh layers in the OSI Reference Model are the Presentation Layer and the Application Layer. The primary purpose of these layers is to facilitate the movement of formatted information between applications interacting with end users on nodes by way of the lower layer protocols. Commonly used top layer protocols are HTTPS (for the secure transfer of web page related files), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP, used for the sending of email messages), and SSH (Secure Shell, used for secure remote shell8 access to a computer operating system).

2.7 SUMMARY OF THE OSI REFERENCE MODEL CONCEPT

The OSI Reference Model exists not to make hard rules or to shape the industry, but to provide a logical, well-researched, and tested model after which the world's best communication protocol "stacks" are modeled. Protocol stacks are a set of two or more protocols that stack on top of each other following the lead of the OSI Reference Model's layered format. The TCP/IP stack is very well-known for being the driving force behind most of the internet, and represents the third (IP) and fourth (TCP) layers of the OSI Model. Every layer in the OSI Model is a reference for a protocol which must facilitate communication between both higher and lower layers. The "U-shaped" example shown in Diagram 4 provides a visual concept of how two users may be linked on a given network in reference to the OSI Model. Data starts and ends with the user. From the Application Layer of the first user, it must travel down through layers 7 to 1, across the transmission medium, then back up layers 1 to 7 to be presented at the Application Layer to the user on the end of the transmission. Diagram 4, of course, only shows an example of a path between two nodes. On node diagrams such as Diagram 1 and Diagram 2, each node is assumed to be operating some stack of OSI based protocols. Protocols defined by this reference are dependent on the next lowest layer protocol. So, for example, one could not run an Application Layer protocol on a node without the presence of Layer 1 through 6 protocols also being utilized on the node. A node could, however, operate with only three layer protocols if it just needed to interact with information in Network Layer (Layer 3) Packets. An example of such a node would be a router, since routers do not need to decipher payload data from layers any higher than Layer 3, the layer which caries the routing information. This stackable concept allows nodes to operate on a scalable range of complexity and capability.

1 Binary; Expressed in a system of numerical notation that has

2 rather than 10 as a base2 meaning 'in binary'

3 Node; A piece of equipment, such as a computer or other device, attached to a network.

4 MAC; Media Access Control. The lower sub-layer of the OSI data link layer. The upper and lower sub-layers of the Data Link Layer are not covered in this document.

5 IPv4 is the standard used for this example. The forthcoming IPv6 uses a different logical addressing scheme, to the same end.

6 IPv4 is the standard used for this example. These figures may not specifically apply in the forthcoming IPv6 standard.

7 Checksum; A digit or digits representing the sum of the correct digits in a piece of stored or transmitted digital data, against which later comparisons can be made to detect errors in the data.

8 The shell of an operating system is a program that presents an interface to various operating system functions and services. The shell is so called because it is an outer layer of interface between the user and the innards of the operating system, the kernel. Source: Wikipedia

Introduction

2008-06-15T19:20:00.002+07:00

1. INTRODUCTION

Over the past forty years computer networks have dramatically changed the way that we work, play, and communicate. In more ways than ever thought possible, we now have information and personal communication literally at our fingertips. The technology has become so evolved and integrated into every area of our lives that most people are not even aware of their lifestyle's significant reliance on computer networks. Television, cellular and landline phones, the Internet, our national military forces, and even the electricity received through the walls of homes would all fail if computer network technology was to vanish from the earth. This document will cover the history, workings, and modern day implementations of computer networks and the most common data communication protocols they utilize.

1.1 HISTORY OF THE CONCEPT

The Oxford American Dictionary primarily defines the word network as "An arrangement of intersecting horizontal and vertical lines; A group of interconnected people or things". One of the simplest concepts developed by man, a network is the infrastructure that allows the efficient linking of multiple points. This term was not coined with the dawn of the computer, but rather closer to the dawn of time. Some well-known examples of networks before the digital age are interconnected irrigation canals, automobile highway systems, and utility delivery lines. Though not commonly called networks, all of these systems can be defined as networks and are excellent examples of a concept that is not new, but rather has found startling new applications in the past four to six decades. The first computers were extremely expensive, room sized machines which required a team of people to operate and performed what, by today's standards, would seem trivial computations. Despite the computer's start being so primitive, it did not take long for the first computer scientists to begin envisioning the potential power of applying the concept of networking to the new world of digital processing. In 1962, a man by the name of Licklider conceived an idea he called the "Galactic Network", and specifically exampled how it could be used for social interactions. Though his concepts were slightly ahead of their time, over the next few decades the idea that he drafted would come into reality.

2008-06-15T19:14:00.001+07:00

1. INTRODUCTION
1.1 History of the Concept

2. THE OSI REFERENCE MODEL
2.1 OSI Layer 1: The Physical Layer
2.2 OSI Layer 2: The Data Link Layer
2.3 OSI Layer 3: The Network Layer
2.4 OSI Layer 4: The Transport Layer
2.5 OSI Layer 5: The Session Layer
2.6 OSI Layers 6 & 7: The Presentation & Application Layers
2.7 Summary of the OSI Concept

3. COMMON NETWORK HARDWARE & INFRASTRUCTURE STANDARDS
3.1 Ethernet (IEEE 802.3)
3.2 Wi-Fi (IEEE 802.11x)
3.3 Bluetooth (IEEE 802.15.1)

4. THE INTERNET PROTOCOL SUITE: A LESSON IN PROTOCOL STACKS
4.1 Introduction to the Internet Protocol Suite
4.2 The Internet Protocol Suite Link Layer
4.3 The Internet Protocol Suite Internetwork Layer
4.4 The Internet Protocol Suite Transport Layer
TCP
UDP
RTP
4.5 The Internet Protocol Suite Application Layer
The Uniform Resource Locator Concept
HTTP
HTTPS
FTP
SSH
4.6 Summary of the Internet Protocol Suite

5. THE INTERNET: THE ULTIMATE NETWORK

6. CONCLUSION

7. SOURCES

Computer networking

2008-06-15T19:05:00.003+07:00

Computer networking is the engineering discipline concerned with communication between computer systems or devices. Networking, routers, routing protocols, and networking over the public Internet have their specifications defined in documents called RFCs.[1] Computer networking is sometimes considered a sub-discipline of telecommunications, computer science, information technology and/or computer engineering. Computer networks rely heavily upon the theoretical and practical application of these scientific and engineering disciplines.
A computer network is any set of computers or devices connected to each other with the ability to exchange data.[2] Examples of networks are:
local area network (LAN), which is usually a small network constrained to a small geographic area.
wide area network (WAN) that is usually a larger network that covers a large geographic area.
wireless LANs and WANs (WLAN & WWAN) is the wireless equivalent of the LAN and WAN
All networks are interconnected to allow communication with a variety of different kinds of media, including twisted-pair copper wire cable, coaxial cable, optical fiber, and various wireless technologies.[3] The devices can be separated by a few meters (e.g. via Bluetooth) or nearly unlimited distances (e.g. via the interconnections of the Internet [4]).

Contents
1 Views of networks
2 History
3 Networking methods
3.1 Local area network (LAN)
3.2 Wide area network (WAN)
3.3 Wireless networks (WLAN, WWAN)
4 Network topology
5 Suggested topics
6 References
7 External links

Views of networks
Users and network administrators often have different views of their networks. Often, users share printers and some servers form a workgroup, which usually means they are in the same geographic location and are on the same LAN. A community of interest has less of a connotation of being in a local area, and should be thought of as a set of arbitrarily located users who share a set of servers, and possibly also communicate via peer-to-peer technologies.
Network administrators see networks from both physical and logical perspectives. The physical perspective involves geographic locations, physical cabling, and the network elements (e.g., routers, bridges and application layer gateways that interconnect the physical media. Logical networks, called, in the TCP/IP architecture, subnets , map onto one or more physical media. For example, a common practice in a campus of buildings is to make a set of LAN cables in each building appear to be a common subnet, using virtual LAN (VLAN) technology.
Both users and administrators will be aware, to varying extents, of the trust and scope characteristics of a network. Again using TCP/IP architectural terminology, an intranet is a community of interest under private administration usually by an enterprise, and is only accessible by authorized users (e.g. employees) (RFC 2547). Intranets do not have to be connected to the Internet, but generally have a limited connection. An extranet is an extension of an intranet that allows secure communications to users outside of the intranet (e.g. business partners, customers)RFC 3547.
Informally, the Internet is the set of users, enterprises,and content providers that are interconnected by Internet Service Providers (ISP). From an engineering standpoint, the Internet is the set of subnets, and aggregates of subnets, which share the registered IP address space and exchange information about the reachability of those IP addresses using the Border Gateway Protocol. Typically, the human-readable names of servers are translated to IP addresses, transparently to users, via the directory function of the Domain Name System (DNS).
Over the Internet, there can be business-to-business (B2B), business-to-consumer (B2C) and consumer-to-consumer (C2C) communications. Especially when money or sensitive information is exchanged, the communications are apt to be secured by some form of communications security mechanism. Intranets and extranets can be securely superimposed onto the Internet, without any access by general Internet users, using secure Virtual Private Network (VPN) technology.
When used for gaming one computer will have to be the server while the others play through it.

History
Before the advent of computer networks that were based upon some type of telecommunications system, communication between calculation machines and early computers was performed by human users by carrying instructions between them. Many of the social behavior seen in today's Internet was demonstrably present in nineteenth-century telegraph networks, and arguably in even earlier networks using visual signals. [5]
In September 1940 George Stibitz used a teletype machine to send instructions for a problem set from his Model K at Dartmouth College in New Hampshire to his Complex Number Calculator in New York and received results back by the same means. Linking output systems like teletypes to computers was an interest at the Advanced Research Projects Agency (ARPA) when, in 1962, J.C.R. Licklider was hired and developed a working group he called the "Intergalactic Network", a precursor to the ARPANet.
In 1964, researchers at Dartmouth developed the Dartmouth Time Sharing System for distributed users of large computer systems. The same year, at MIT, a research group supported by General Electric and Bell Labs used a computer (DEC's PDP-8) to route and manage telephone connections.
Throughout the 1960s Leonard Kleinrock, Paul Baran and Donald Davies independently conceptualized and developed network systems which used datagrams or packets that could be used in a packet switched network between computer systems.
1965 Thomas Merrill and Lawrence G. Roberts created the first wide area network(WAN).
The first widely used PSTN switch that used true computer control was the Western Electric 1ESS switch, introduced in 1965.
In 1969 the University of California at Los Angeles, SRI (in Stanford), University of California at Santa Barbara, and the University of Utah were connected as the beginning of the ARPANet network using 50 kbit/s circuits. Commercial services using X.25, an alternative architecture to the TCP/IP suite, were deployed in 1972.
Computer networks, and the technologies needed to connect and communicate through and between them, continue to drive computer hardware, software, and peripherals industries. This expansion is mirrored by growth in the numbers and types of users of networks from the researcher to the home user.
Today, computer networks are the core of modern communication. For example, all modern aspects of the Public Switched Telephone Network (PSTN) are computer-controlled, and telephony increasingly runs over the Internet Protocol, although not necessarily the public Internet. The scope of communication has increased significantly in the past decade and this boom in communications would not have been possible without the progressively advancing computer network.

Networking methods
Networking is a complex part of computing that makes up most of the IT Industry. Without networks, almost all communication in the world would cease to happen. It is because of networking that telephones, televisions, the internet, etc. work.
One way to categorize computer networks is by their geographic scope, although many real-world networks interconnect Local Area Networks (LAN) via Wide Area Networks (WAN)and wireless networks[WWAN]. These three (broad) types are:

Local area network (LAN)
A local area network is a network that spans a relatively small space and provides services to a small number of people. Depending on the number of people that use a Local Area Network, a peer-to-peer or client-server method of networking may be used. A peer-to-peer network is where each client shares their resources with other workstations in the network. Examples of peer-to-peer networks are: Small office networks where resource use is minimal and a home network. A client-server network is where every client is connected to the server and each other. Client-server networks use servers in different capacities. These can be classified into two types: Single-service servers, where the server performs one task such as file server, print server, etc.; while other servers can not only perform in the capacity of file servers and print servers, but they also conduct calculations and use these to provide information to clients (Web/Intranet Server). Computers are linked via Ethernet Cable, can be joined either directly (one computer to another), or via a network hub that allows multiple connections.
Historically, LANs have featured much higher speeds than WANs. This is not necessarily the case when the WAN technology appears as Metro Ethernet, implemented over optical transmission systems.

Wide area network (WAN)
A wide area network is a network where a wide variety of resources are deployed across a large domestic area or internationally. An example of this is a multinational business that uses a WAN to interconnect their offices in different countries. The largest and best example of a WAN is the Internet, which is a network comprised of many smaller networks. The Internet is considered the largest network in the world.[6]. The PSTN (Public Switched Telephone Network) also is an extremely large network that is converging to use Internet technologies, although not necessarily through the public Internet.
A Wide Area Network involves communication through the use of a wide range of different technologies. These technologies include Point-to-Point WANs such as Point-to-Point Protocol (PPP) and High-Level Data Link Control (HDLC), Frame Relay, ATM (Asynchronous Transfer Mode) and Sonet (Synchronous Optical Network). The difference between the WAN technologies is based on the switching capabilities they perform and the speed at which sending and receiving bits of information (data) occur.
For more information on WANs, see Frame Relay, ATM and Sonet.

Wireless networks (WLAN, WWAN)
A wireless network is basically the same as a LAN or a WAN but there are no wires between hosts and servers. The data is transferred over sets of radio transceivers. These types of networks are beneficial when it is too costly or inconvenient to run the necessary cables. For more information, see Wireless LAN and Wireless wide area network. The media access protocols for LANs come from the IEEE.
The most common IEEE 802.11 WLANs cover, depending on antennas, ranges from hundreds of meters to a few kilometers. For larger areas, either communications satellites of various types, cellular radio, or wireless local loop (IEEE 802.16) all have advantages and disadvantages. Depending on the type of mobility needed, the relevant standards may come from the IETF or the ITU.

Network topology
The network topology defines the way in which computers, printers, and other devices are connected, physically and logically. A network topology describes the layout of the wire and devices as well as the paths used by data transmissions. Commonly used topologies include:
Bus
Star
Tree (hierarchical)
Linear
Ring
Mesh
partially connected
fully connected (sometimes known as fully redundant)
The network topologies mentioned above are only a general representation of the kinds of topologies used in computer network and are considered basic topologies.

Computer Networks and Protocol

ระบบคอมพิวเตอร์ (Computer System)

Very Easy Home Network

Why Use Ethernet Routers

Cisco CBAC – The Poor Mans Firewall

Home Computer Networking

Networking (Computers)

Why Use Ethernet Routers

IP Addresses, Subnetting and Sub-subnetting

IP Address Classes

Network Design Manual

Introduction to TCP/IP

Wireless LAN

LAN TOPOLOGY

Protocol (computing)

protocol (network)

Conclusion

The Internet: The Ultimate Network

The Internet Protocol Suite:

Common Network Hardware

The OSI Reference Model

Introduction

TABLE OF CONTENTS

Computer networking