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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)
                9  Internet Protocol Version 4 (IP, IPv4)
                     9  IP Addressing

Previous Topic/Section
IP Subnetting Step #5: Determining Host Addresses For Each Subnet
Previous Page
Pages in Current Topic/Section
1
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IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages
Next Topic/Section

IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting"

As the early Internet began to grow dramatically, three 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 Routing (CIDR).

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.


Quick navigation to subsections and regular topics in this section



Previous Topic/Section
IP Subnetting Step #5: Determining Host Addresses For Each Subnet
Previous Page
Pages in Current Topic/Section
1
Next Page
IP Classless Addressing and "Supernetting" Overview, Motivation, Advantages and Disadvantages
Next Topic/Section

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Version 3.0 - Version Date: September 20, 2005

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