2.2 Classless Interdomain Routing
2.2.2 Classless interdomain routing features
Classless Addressing

Mathematically, the IPv4 address space still held a substantial number of available addresses. Unfortunately, many of these potential addresses were squandered because they were locked into assigned blocks, or classes, of assigned addresses. Eliminating classes wouldn't necessarily recover the addresses locked into those address spaces that were already assigned, but it would enable the remaining addresses to be used much more efficiently. Ostensibly, this stopgap effort would buy the time needed for IPv6 to be developed and deployed.

Enhanced Route Aggregation

CIDR enables Internet routers (or any CIDR-compliant router) to more efficiently aggregate routing information. In other words, a single entry in a routing table can represent the address spaces of many networks. This can greatly reduce the size of the routing tables that are needed in any given internetwork and directly translates into an increased scalability.

CIDR was implemented in the Internet during 1994 through 1995 and was immediately effective in containing the expansion of the Internet routers' routing tables. It is doubtful that the Internet would have continued to grow had CIDR not been implemented.

Supernetting

Another benefit of CIDR is the capability to supernet. Supernetting is nothing more than using contiguous blocks of Class C address spaces to simulate a single, albeit larger address space. If you were to obtain enough contiguous Class C addresses, you could redefine the allocation of bits between network and host identification fields and simulate a Class B address.

Supernetting is designed to alleviate the pressure on the rapidly depleting Class B address space by offering a more flexible alternative. The previous class-based address architecture suffered from a tremendous disparity between its Class B and C networks. Networks that required more than the 254 hosts offered by a Class C had the following two choices, neither of which was highly desirable:

  1. Using multiple Class C addresses (which would have necessitated routing between the network domains)
  2. Stepping up to a Class B address with its 65,534 usable host addresses

The simpler solution, frequently, was to use the Class B even though it wasted tens of thousands of IP addresses.

Consider a company (Catco, Inc.) that has been granted the following registered Class C addresses: 220.220.1.0 through 220.220.255.0. Assume that Catco has assigned (used) 220.220.1.0 through 220.220.100.0. Catco does not need to advertise the entire list of 100 individual Class C addresses to the Internet. Instead, Catco can supernet the addresses to a shorter prefix: 220.220.0.0/16. This results in a 99 percent reduction in the number of routes that need to be advertised to the Internet. Full connectivity from the Internet to the 100 assigned Class C addresses is still possible because Catco has control of every possible network with the 16-bit supernet prefix of 220.220.0.0 (220.220.1.0, 220.220.2.0, . . . , 220.220.254.0 and 220.220.255.0).