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

IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting" 1 2 3 IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation

A Better Solution: Eliminate Address Classes

It was clear that as long as there were only three sizes of networks, the allocation efficiency problem could never be properly rectified. The solution was to get rid of the classes completely, in favor of a classless allocation scheme. This system would solve both of the main problems with “classful” addressing: inefficient address space use, and the exponential growth of routing tables.

This system was developed in the early 1990s and formalized in 1993 in RFCs 1517, 1518, 1519 and 1520. The technology was called Classless Inter-Domain Routing (CIDR). Despite this name, the scheme deals with both addressing and routing matters, since they are inextricably linked.

The idea behind CIDR is to adapt the concept of subnetting a single network to the entire internet. In essence, then, classless addressing means that instead of breaking a particular network into subnets, we can aggregate networks into larger “supernets”. CIDR is sometimes called supernetting for this reason: it applies the principles of subnetting to larger networks. It is this aggregation of networks into supernets that allowed CIDR to resolve the problem of growing Internet routing tables.

Of course, if we are going to apply subnetting concepts to the entire internet, we need to be able to have subnets of different sizes. After all, that's one of our primary goals in eliminating the classes. So, more accurately, CIDR is an internet-wide application of not regular one-level subnetting, but of Variable Length Subnet Masking (VLSM). Just as VLSM lets us split a network as many times as we want to create subnets, “sub-subnets” and “sub-sub-subnets”, CIDR lets us do this with the entire Internet, as many times as needed.

IP Classless Addressing: Classless Inter-Domain Routing (CIDR) / "Supernetting" 1 2 3 IP "Supernetting": Classless Inter-Domain Routing (CIDR) Hierarchical Addressing and Notation