CIDR (Classless Inter-Domain Routing)

Classless Inter-Domain Routing (CIDR) is the method used to allocate and route IP addresses on the internet and in private networks. Introduced in 1993 through RFC 1519, and later revised by RFC 4632, CIDR replaced the rigid classful addressing scheme that divided the IPv4 address space into fixed-size blocks. By letting prefix lengths fall on any bit boundary, CIDR made address allocation far more efficient and enabled the route aggregation that keeps the global routing table manageable.

Notation

A CIDR block is written as an address followed by a slash and a prefix length, eg. 192.168.1.0/24. The prefix length counts the leading bits that identify the network. The remaining bits identify individual hosts within it. A /24 leaves 8 bits for hosts, yielding 256 addresses (254 usable once the network and broadcast addresses are reserved). Prefix lengths range from /0, which covers the entire address space, to /32 for IPv4 or /128 for IPv6, each of which identifies a single host.

What CIDR replaced

Under the classful system that preceded CIDR, an organization needing 500 addresses had to take a Class B block of 65,536 addresses, wasting over 65,000 of them, or stitch together two Class C blocks of 256 each, which added two separate entries to every routing table. CIDR lets a registrar hand that organization a single /23 block of 512 addresses, sized to the actual need.

Route aggregation

CIDR’s most consequential property is route aggregation, also called supernetting. Because prefixes can start on any bit boundary, a router can announce one short prefix that covers many longer, more specific prefixes beneath it. An internet service provider assigned 203.0.113.0/24 can split it into /25 subnets for individual customers, yet advertise only the single /24 to the rest of the internet. Many specific routes collapse into one less specific announcement.

Without this, the global routing table would have grown past what routers could hold. BGP advertises CIDR prefixes, and the aggregation they enable is what allowed the internet to keep growing past the limits of the classful system.

Subnetting

The flip side of aggregation is subnetting: splitting a prefix into smaller prefixes to segment a network. A /24 can be divided into two `/25`s, four `/26`s, and so on, each a self-contained subnet. This is how organizations partition a single address block across teams, floors, or security zones without requesting more addresses. CIDR’s variable-length subnet masking (VLSM) lets each subnet be sized to its actual need, rather than forcing every subnet to the same fixed mask.

See also

• Internet Protocol (IP)
• IP address
• Border Gateway Protocol (BGP)

References

• Fuller, V., Li, T., Yu, J., and Varadhan, K. (1993). Classless Inter-Domain Routing (CIDR): an Address Assignment and Aggregation Strategy. RFC 1519, IETF.
• Fuller, V. and Li, T. (2006). Classless Inter-Domain Routing (CIDR): The Internet Address Assignment and Aggregation Plan. RFC 4632, IETF.
• Amazon Web Services (n.d.). What is CIDR?. AWS.