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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:

  • 123.99.86.255/32

32 = 1 IP address: 123.99.86.255/32 represents the IP address 123.99.86.255

  • 123.99.86.254/31

31 = 2 IP addresses: 123.99.86.254/31 represents the IP addresses 123.99.86.254 and 123.99.86.255

  • 123.99.86.252/30

30 = 4 IP addresses: 123.99.86.252/30 represents the IP addresses 123.99.86.252 through 123.99.86.255

  • 123.99.86.0/24

24 = 256 IP addresses: 123.99.86.0/24 represents all of the IP addresses in the range of 123.99.86.0 through 123.99.86.255

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