Classless Inter-Domain Routing (CIDR)


What is CIDR?

Classless Inter-Domain Routing (CIDR), also called supernetting, is a way to more flexibly allocate Internet Protocol (IP) addresses by creating unique and more granular identifiers for networks and individual devices. It was introduced in 1993 as an alternative to Internet routers that managed network traffic based on the class of IP addresses and determined subnetworks, for routing, based on IP address class.

The objective of CIDR was to address scalability issues with classful IP addresses which are based on three classes – Class A, Class B, and Class C. It is the capacity of each IP address class that creates scalability issues. Class A capacity is 16,581,375 IP addresses; Class B is 65,536 IP addresses; and Class C is 256 IP addresses. Using classful addressing led to inefficiencies in address use and routing, because of the rigid limitations of the classes (e.g., if 300 addresses were needed, Class B would be required leaving 16,281 unused). CIDR allows IP addresses to be variable and not bound by the size limitations of Classes A, B, and C.

Since it is not bound by Class, CIDR can organize IP addresses into subnetworks independent of the value of the addresses themselves. This is referred to as supernetting because CIDR effectively allows the aggregation of multiple subnets into a supernet for network routing. With this alternative to traditional subnetting, it is possible to specify the number of significant bits that make up the routing or networking portion by adding this to the IP address. This not only reduces wasted address space but also provides a flexible way to specify network addresses in routers.

Classless IP addresses, enabled by CIDR, are required when creating a Virtual Private Cloud (VPC) that is logically isolated from other virtual networks. When creating a VPC, a range of IPv4 addresses must be specified in the form of a CIDR.