CIDR (Classless Inter-domain routing sometime called suppernetting) is a way to allow more flexible allocation of Internet protocol (IP) addresses than was possible with the original system of IP address classes. As a result number of available internet address was greatly increased which along with widespread use of network address translation (NAT).

Originally IP address were assigned in four major address classes A through D each of these classes allocates one portion of the 32 bit IP address format to identify a network gateway the first 8 bits for class A, the fixed 16 bits for class B and the first 24 for class C. The remainder to identify hosts on that network more than 16 million in class A, 65535 in class B and 254 in class C, class D address identify multicast domains.

Consider that one of the most commonly used classes was class B, An organization that needed more than 254 host machine would often get a class B license even though it would have for fewer than 65534 hosts, this resulted in most of the block of address allocated going unused.

CIDR reduced the problem of wasted address space by providing a new and more flexible way to specify network address in routers, CIDR lets one routing table entry represent an aggregation of networks that exist in forward path that don’t need to be specified on that particular gateway this aggregation of network in a single address is sometime reffered to as a supernet.

Example:- How the public telephone system uses area codes to channel calls toward a certain part of the network. Using CIDR each IP address has a network prefix that identifies either one or several network gateways, the length of the network prefix in IPV4 CIDR is also specified as part of the IP address and varies depending on the number of bits needed. CIDR network address looks like under IPV4.

192.30.250.00/18

The 192.30.250.0 is the network address itself and the “18” says that the first 18 bits arer the network part of the address, leaving the last 14 bits for specific host address.