Classless InterDomain Routing (CIDR)

Classless InterDomain Routing, or CIDR, is a way of grouping networks together for routing purposes across the Internet between domains. With the IPv4 address space running out, CIDR makes much more effective use of the available addresses. CIDR is also known as supernetting , where a number of network addresses are treated as a single address (as opposed to subnetting , where a single network address is divided into smaller networks).

CIDR works by using classless addresses, unlike IPv4, which uses classful addressing. For routing purposes, a single classless address can represent an aggregation of networks (in the same way that the telephone system uses area codes). A CIDR network address looks like this:

 192.15.145.0/18 

where 192.15.145.0 represents the network address itself and the 18 shows that the first 18 bits of the address are the network part of the address. The remaining 14 bits of the 32-bit IPv4 address are for specific host addresses.

CIDR uses netmasks ”which are referred to as network prefixes ”to create various network sizes, similar to the way that subnetworking uses network masks. The 18-bit network portion shown earlier is equivalent to a netmask of 255.255.192.0 .