edVirtus Reference

What Is Classless Inter-Domain Routing?

How Does CIDR Improve the Allocation and Routing of IP Addresses?

Classless Inter-Domain Routing (CIDR) is the addressing method used by modern Internet Protocol (IP) networks to allocate IP address blocks and summarise routing information efficiently. Introduced in 1993, CIDR replaced the original class-based addressing system, allowing Internet addresses to be allocated in blocks of almost any size rather than being restricted to the fixed Class A, B, and C network sizes.

The basic principle is straightforward. Rather than identifying a network by its address class, CIDR specifies how many bits of an IP address represent the network portion by appending a prefix length after a slash. For example, the address block 192.168.10.0/24 indicates that the first 24 bits identify the network and the remaining 8 bits identify individual hosts. Likewise, a /16 prefix provides a much larger network, while a /28 prefix provides only a small number of host addresses.

A useful analogy is organising postal deliveries by postcode. Instead of dividing a country into only a few very large delivery districts, the postcode system allows areas of many different sizes to be defined as required. Similarly, CIDR allows IP address blocks to be allocated according to actual need rather than forcing organisations to accept much larger address ranges than necessary.

One of the principal advantages of CIDR is its efficient use of the limited IPv4 address space. Under the original class-based system, an organisation requiring only a few hundred addresses might have been allocated an entire Class B network containing more than 65,000 addresses, wasting most of them. CIDR allows Internet service providers to allocate address blocks much more precisely, significantly reducing address wastage.

CIDR also enables route aggregation, sometimes called supernetting. Multiple contiguous address blocks can be represented by a single routing table entry. For example, the four adjacent networks 192.168.0.0/24 through 192.168.3.0/24 can be advertised as the single aggregated route 192.168.0.0/22. This substantially reduces the size of Internet routing tables, allowing routers to operate more efficiently and improving the scalability of the global Internet.

It is important to distinguish CIDR from the original classful addressing system. Classful addressing restricted networks to predefined Class A, B, or C boundaries, whereas CIDR permits the network prefix to end at any bit position. Likewise, CIDR differs from subnetting. Traditional subnetting divides an existing network into smaller networks, while CIDR provides the flexible address allocation mechanism that makes modern subnetting and route aggregation possible.

Today, Classless Inter-Domain Routing forms the foundation of IPv4 addressing and routing throughout the Internet. Together with dynamic routing protocols such as OSPF and BGP, CIDR enables scalable network design, efficient address allocation, and manageable routing tables, making it one of the key technologies that allowed the Internet to grow to its present global scale.