CIDR : Classless Inter-domain Routing

The growing demand for IP addresses has put a strain on the classful model, especially class B address space, which was getting depleted at a fast pace. One of the measures to handle the IP depletion is the introduction of CIDR (Classless Inter-domain Routing).

CIDR is a move away from traditional IP classes A/B/C. In CIDR, an IP network is represented by a prefix, which is an IP address and some indication of the left-most contiguous significant bits within this address. CIDR uses extended network prefixes between the left-most 13 and 27 bits, comparing with the traditional class A/B/C structure which is limited to 8/16/24 bits respectively.

For example, used to be an illegal class C network, is now a valid prefix with a notation The /16 is an indication of using 16 bits of mask counting from the far left. A network is called a supernet when the prefix boundary contains fewer bits than the network’s natural mask.


  CIDR Block Prefix Equivalent Class Number of Hosts
  /27 1/8 of Class C 32
  /26 1/4 of Class C 64
  /24 1 Class C 256
  /16 1 Class B 65,536
  /13 8 Class B 524,288


In recent years, the IP routing tables held in the Internet routers have grown in a way that caused routers to start being saturated as far as processing power and memory allocation. CIDR is the technique supported by BGP4 (Border Gateway Protocol version 4) and based on route aggregation. CIDR allows routers to group routes together in order to cut down on the quantity of routing information carried by the core routers. With CIDR, several IP networks appear to networks outside the group as a single, larger entity.

Routing domains that are CIDR-capable are called classless domains, in contract to the traditional classful routing domains. CIDR has depicted a more hierarchical Internet architecture, where each domain takes its IP addressed from a higher hierarchical level. This gives tremendous savings in route propagation especially when summarisation is done close to the so-called leaf networks. Leaf networks are endpoints on the global network; they do not provide Internet connection to other networks. An ISP that supports numerous leaf networks subdivides its subnets into many smaller blocks of addresses to serve their customers. Aggregation permits the ISP to advertise the addresses in a single notation rather than many, thus resulting in more efficient routing strategies and propagation.


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