Thread: Cidr

CIDR stands for classless inter-domain routing.
Previously IP addresses were managed by organisations on the basis of classes.
There are mainly four classes of IP addresses (Class A-D)
Each class specifies number of hosts that can be present on the network. Class B was the most used one. e.g.
Class # Network Bits # Hosts Bits Subnet mask
Class B 16 bits 16 bits 255.255.0.0

A class B address means maximum number of 2^16=655535 host addresses for each N/w address. But the Classes used meant wastage of IP addresses.

Therefore CIDR was introduced in which network bits can be assigned values according to usage e.g. 172.21.160.0/24 means that network address is 172.21.160.0 and hosts range from 172.21.160.1-172.21.160.255. BGP/OSPF/ISIS supports CIDR.