Classless Interdomain Routing


VLSM helps improve the efficiency of IP address usage for an assigned address block; however, it does not solve challenges with inefficient allocation of addresses to organiza-tions. The imminent depletion of IP addresses as the result of inefficient use of classful blocks and the growing number of classful addresses in the global Internet routing tables as organizations were allocated multiples of a Class C address instead of a single Class B address led to the introduction of classless interdomain routing (CIDR).

CIDR allows an IP network number to be any length, abandoning completely the fixed boundaries associated with classful concepts. The two benefits of CIDR are illustrated in the examples provided in Figure 1-6. By eliminating the notions of address classes, a block of addresses such as 192.168.0.0 to 192.168.255.0 consisting of an individual Class C address can be considered a uniform block that can be conveniently represented as 192.168.0.0/16. This essentially implies aggregation of 256 "old notion" Class C addresses into a single address block, referred to as a CIDR block or a supernet.

Figure 1-6. Examples of CIDR Aggregation and Subnetting

Examples of CIDR Aggregation and Subnetting

CIDR also allows network numbers to be flexibly subnetted and allocated to different organizations for interdomain routing exchange. For example, 131.108.0.0/16 can be divided into four subblocks (131.108.0.0/18, 131.108.64.0/18, 131.108.128.0/18, and 131.108.192.0/18) and allocated to four different organizations instead of one.