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.
32 = 1 IP address: 18.104.22.168/32 represents the IP address 22.214.171.124
31 = 2 IP addresses: 126.96.36.199/31 represents the IP addresses 188.8.131.52 and 184.108.40.206
30 = 4 IP addresses: 220.127.116.11/30 represents the IP addresses 18.104.22.168 through 22.214.171.124
24 = 256 IP addresses: 126.96.36.199/24 represents all of the IP addresses in the range of 188.8.131.52 through 184.108.40.206