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.