• noun a system of organising IP addresses that is more compact and efficient than the older system, adding a slash and a new IP prefix number that represents a number of individual addresses. For example the old system used an IP address such as ‘’ whereas CIDR would replace this with ‘’. The IP Prefix number 12 represents 4,096 unique addresses, and the lower the number the more addresses are represented.


  • acronym forclassless inter-domain routing
  • A technique in which routers group routes together so as to reduce the information carried by the core routers. Also spelled classless interdomain routing. Its abbreviation is CIDR.