What is CIDR?

CIDR stands for “Classless Inter-Domain Routing” and is also known as subnetting. The original goal of CIDR was to slow down the exhaustion of IPv4 addresses. However, now it’s mainly a method for summarizing IP addresses and improving the efficiency of IPv4 assignments.

Fun fact: CIDR is pronounced as CIDER.

The history of CIDR

Before we dive deeper into CIDR, we need to understand what brought us to this method.

It all starts in the early 1990s when we had “Class-Based IP addresses”. With Class-Based IP addresses, there were three block sizes, which were also known as “Classes”:

  • A Class A network/block had 16,777,216 addresses
  • A Class B network/block had  65,536 addresses
  • A Class C network/block had 256 addresses

If a company with a Class A block needed more than 256 addresses, they would be switched to a Class B network. However, this could waste over 60,000 addresses if the company wouldn’t use them all.

Being limited to only three Classes, the classfull design drained the pool of unassigned IPv4 addresses faster than necessary. Technical engineers soon realized that we were starting to run out of IPv4 addresses a little too quickly. To solve this issue, a more specific allocation was required. And so we moved to a classless IPv4 system in 1993.

Introducing the CIDR structure

The CIDR structure was introduced by the Internet Engineering Task Force (IETF) and allows Internet Engineers to divide IP addresses into subnets. Addresses with a CIDR structure have two sets of numbers:

  1. The prefix – this is a binary representation of a networks address
  2. The suffix – this is the total number of bits in an address

A CIDR block is a group of IPv4 addresses that share the same prefix and contain the same number of bits.  An example of a CIDR address is: This network has /24 bits.

IPv4 addresses can have up to 32 bits. However, IPv6 addresses, which are planned to replace IPv4 addresses in the future, also have /64 and /48 prefixes. In fact, IPv6 addresses can go up to 128 bits.

The Benefit of CIDR: Subnetting

CIDR makes it possible to make the most of your IPv4 addresses through subnetting. Through subnetting, addresses can be partitioned. For example, an ISP that provides services for homes might create a separate network for every home. Specifically, your home ISP may have assigned you one address (a /32 network) but may summarize all of its customers as one or more /16 supernets of all the addresses. These can be further broken down into /24 subnets. 


Because of CIDR, fewer unused IPv4 addresses are wasted and IPv4 addresses can be allocated more effectively.

Click here if you’d like to see our IPv4 CIDR chart, or read this blog if you’d like to learn more about the differences between IPv4 and IPv6 addresses.

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