CIDR

Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and for routing Internet Protocol packets. It was introduced in 1993 to slow the growth of routing tables and to extend the usable life of IPv4 by replacing the older classful addressing system. CIDR also enables more flexible subnetting and address management.

CIDR uses a slash notation to designate a network prefix, such as 192.0.2.0/24. In this notation, the number after the slash indicates the length of the fixed network portion of the address in bits. The remaining bits identify hosts within the network. CIDR prefixes can vary in size, and their lengths are typically chosen to fit the needs of the network.

Blocks defined by CIDR are typically powers of two in size and must be aligned to proper boundaries. For example, a /24 block contains 256 addresses, a /16 block contains 65,536 addresses, and so on. CIDR makes it possible to allocate just the right amount of address space and to combine smaller blocks into larger, aggregate prefixes when routing.

A key feature of CIDR is route aggregation or supernetting, where multiple smaller prefixes are represented as a single, larger prefix in routing tables. This reduces the size of global routing tables and improves scalability. CIDR is used with both IPv4 and IPv6; in IPv6, it is common to see /64 subnets as a standard practice.

IP address allocations are managed by Regional Internet Registries (RIRs), who assign CIDR blocks to Internet Service Providers and organizations. Those networks may further subnet their allocations using CIDR, with ongoing planning to avoid overlaps and ensure efficient routing.