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IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"
As the early Internet began to grow
main problems arose with the original classful addressing
scheme. These difficulties were addressed
partially through subnet
addressing, which provides more flexibility
for the administrators of individual networks on an internet. Subnetting,
however, doesn't really tackle the problems in general terms. Some of
these issues remain due to the use of classes even with subnets.
While development began on IP
version 6 and its roomy
128-bit addressing system in the mid-1990s,
it was recognized that it would take many years before widespread deployment
of IPv6 would be possible. In order to extend the life of IP version
4 until the newer IP version 6 could be completed, it was necessary
to take a new approach to addressing IPv4 devices. This new system calls
for eliminating the notion of address classes entirely, creating a new
classless addressing scheme sometimes called Classless Inter-Domain
In this section I describe modern
classless IP addressing. I begin with an overview of the concepts behind
classless addressing and the idea behind supernetting, including
why it was created and what its advantages and disadvantages are. I
then define CIDR and describe how the system works in more detail, including
the notation used for address blocks. I list each of the CIDR address
block sizes and show how they relate to the older class A, B and C networks.
I conclude with an example of CIDR addressing, which is similar to the
practical subnetting section prior to this one, but focused on CIDR
and a bit more condensed.
Background Information: Classless IP addressing represents the latest evolution of IP addressing, following on the heels of subnetting and the original classful addressing system described in preceding sections. Understanding classless IP addressing and routing requires at least some familiarity with these older IP addressing methods. If you have come to this section without reading the preceding sections on classful addressing and subnetting, I strongly advise reviewing them first. If you understand subnetting but aren't familiar with how Variable Length Subnet Masking (VLSM) works, reading the topic on VLSM is a good idea, since CIDR is similar to VLSM in many ways.
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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