Classless interdomain routing (CIDR) is a system of addressing that improves the scaling factor of routing in the Internet. CIDR does not use an implicit mask based on the class of network. In CIDR, an IP network is represented by a prefix, which is an IP address and an indication of the leftmost contiguous significant bits within this address.
For example, without CIDR, the class C network address 192.56.0.0 would be an illegal address. With CIDR, the address becomes valid with the notation: 192.56.0.0/16. The /16 indicates that 16 bits of mask are being used (counting from the far left). This would be similar to an address 198.32.0.0. with a mask of 255.255.0.0.
A network is called a supernet when the prefix boundary contains fewer bits than the network’s natural mask. For example, a class C network 192.56.10.0 has a natural mask of 255.255.255.0. The representation 192.56.0.0/16 has a shorter mask than the natural mask (16 is less than 24), so it is a supernet.
Figure 5 shows how CIDR can reduce the number of entries globally in Internet routing tables. A service provider has a group of customers with class C addresses that begin with 192.56. Despite this relationship, the service provider announces each of the networks individually into the global Internet routing mesh.
Figure 5: Routing With and Without CIDR
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