Sunday, July 27, 2008

ระบบคอมพิวเตอร์ (Computer System)

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

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

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

Wednesday, July 2, 2008

Very Easy Home Network

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

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

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

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)

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

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

Sunday, June 15, 2008

IP Addresses, Subnetting and Sub-subnetting

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:

  1. 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.
  2. 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.
  3. 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

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

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

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:

  1. The IP address assigned to this personal computer
  2. 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)
  3. 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.

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

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


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