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Table Of Contents  CertiGuide to Network+
 9  Chapter 0101:  TCP/IP

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XV  Network Address Translation
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XVI  Classless Inter-Domain Routing

Before examining what Classless Inter-Domain Routing (CIDR) is, a fleshing out of Class based IP addresses will help. Looking at the Class A address, you are aware that it is a private address, and the subnet mask would be

The subnet mask is a set of four numbers, similar to an IP address, which indicate the number of bits of the address that are the network number. We’ll briefly review subnet masks below. If you’re new to the concept, you may also want to review Appendix B, which offers a more detailed discussion of IP address subnetting and subnet masks.

Want an easier way to write that address? Try

Since the subnet mask is really 11111111000000000000000000000000

Why not just count the ones and say /8, to indicate that the network ID is contained in the first 8 bits of the 32-bit IP address?

It sure is simpler to write.

But wait! There’s more!

Sometimes it helps in larger LAN environments to segment traffic by physical wire, and still allow communications across the entire network. This is frequently called custom sub netting. The classic custom sub nets are 192, 224, 240, 248,252, and 254.

It starts at 192 because the rules (RFC’s) required that in a custom subnet, the first two digits be on or ones. Thinking back to the math,

128 64 32 16 8 4 2 1

Then, the string of digits, where on is a 1, and off is a 0:

on on off off off off off off

works out to a total of 192.

Therefore a custom subnet mask would look like

Ok, no big deal in an LAN environment. And to quote the RFC that addresses the issue for the whole Internet: “There is a major problem with the use of a range of Class C addresses instead of a single Class B addresses: each network must be routed separately. Standard IP routing understands only the class, A, B, and C network classes.

Within each of these types of networks, subnetting can be used to provided better granularity of the address space within each network, but there is not way to specify that multiple class C network are actually related. The result of this is termed the routing table explosion problem: a Class B network of 3000 hosts requires one routing table entry at each backbone router, but if the name network is addressed as a range of Class C networks, it requires 16 entries.

The solution to the problem is a scheme called “Classesless-InterDomain (sic) Routing (CIDR).” The document continues “CIDR does not route according to the class of the network number (hence the term classless) but solely according to the high order bits of the IP address which are termed the IP prefix.

This example of CIDR shows how TCP/IP was designed to change and grow with unexpected needs. This change “slows the issue of routing table explosion problem”. It does not do anything for the fact that as the entire world moves to a single transport (TCP/IP) and in the ideal world, everything would have a unique IP number. While there was debate that measures such as NAT would make the IP number exhaustion a moot point, time has killed that discussion. We are running out of IP numbers, period. It is no longer an IF question, but a when question. Microsoft thinks it knows when, and has included the solution with the release of Windows XP. UNIX vendors have also been including this enhanced TCP/IP addressing in major versions of UNIX for years. That solution is the next topic.

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Version 1.0 - Version Date: November 7, 2004

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