CIDR (Classless Inter-Domain Routing)

Classless Inter-Domain Routing (CIDR) is a way of allowing many people to share the same address block. Collections of IP addresses are divided into groups. A single group route can cover the address space of several earlier numbers.

URLs:

CIDR FAQ Draft

Frequently Asked Questions about CIDR.

RFC 1520

Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy. CIDR is documented in Request for Comments (RFCs) 1517/1518/1519/1520.

W3E References:

router

CIDR involves the rewriting of router tables.

Detail:

CIDR was implemented in 1994 and 1995 to conserve IP address space, slow the rate of growth in the size of routing tables, and enable the uninterrupted growth of the Internet. In December 1990 there were 2,190 routes; in July, 1995, there were over 29,000 routes. Without CIDRization, the routing tables would have been in excess of 65,000 routes.

The notion of "classless" routing stems from the elimination of the distinction between class A, B, and C networks and their replacement with a generalized IP prefix.