CIDR - Classless Inter-Domain Routing

Definition: CIDR is an efficient method for specifying IP addresses to Internet routers. CIDR was developed to cope with the surge in demand for IPv4 Internet addresses in the 1990s.

Before CIDR, Internet routers used an inefficient IP addressing scheme based on classes. Organizations like ISPs reserved address blocks in large "Class A," "Class B," or "Class C" chunks that wasted much of the IP address range.

In contrast, CIDR makes the IP addressing space classless. CIDR associates network masks with IP network numbers independent of their traditional class. Routers that support CIDR recognize these networks as individual routes, even though they may represent an aggregation of several traditional subnets.

Also Known As: Classless Inter-Domain Routing, Classless Internet Domain Routing, supernetting

Examples:

CIDR shorthand notation writes an IP address and its associated network mask in the form xxx.xxx.xxx.xxx/n, where 'n' is a number between 1 and 31 that is the number of '1' bits in the mask.