What is Classless Inter-Domain Routing (CIDR)?

Classless Inter-Domain Routing (CIDR) is a standard created by the Internet Engineering Task Force in 1993 that helps to more efficiently allocate IP addresses and allows for a flexible and simplified way to identify IP addresses and route network traffic. It is similar to how telephones use:

  • An area code to specify a geographical region or city on the telephone network (for example, 212 for New York)
  • A number to identify a specific person or device (for example, 555-5555)

It uses CIDR notation to simplify how a range of IP addresses are represented. It consists of an IP address, a forward slash (/) and a number that ranges from 0 to 32. The numbers 0 to 32 represent a range of IP addresses. Every time the number decreases by one (starting at 32), it means the number of IP addresses in that range are doubled.

For example:


32 = 1 IP address: represents the IP address


31 = 2 IP addresses: represents the IP addresses and


30 = 4 IP addresses: represents the IP addresses through


24 = 256 IP addresses: represents all of the IP addresses in the range of through

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