• Directory of Internet terms
  • Cidr — Classless Inter-Domain Routing. This is a change in specifying ranges of IP addresses from the old Class A, B, and C address blocks. IP addresses consist of 32 bits, usually expressed in four 8-bit numbers, such as 122.63.101.17. In the old system, Class C specified 24 bits (the first three numbers in the conventional IP address), leaving all the addresses in the remaining 8 bits to be assigned by the registrant (256 addresses, less a few broadcast only addresses). Class B specified 16 bits and class A, 8 bits, leaving the balance to the registrant. Now, the IP address is followed by an IP Prefix, such as 122.63.101.14/26. The /26 IP Prefix means that the first 26 bits of the given IP address are fixed, and the registrant has the remaining 6 bits (64 addresses) to use and assign. IP Prefixes are currently used ranging from /27 (25 or 32 addresses) to /13 (219 or 524,288 addresses.