Posted in Networking

Classless Interdomain Routing (CIDR)

You can think of the Internet as one big network. As such, each device on the network needs its own unique IP address. In the early days of the Internet, every device would receive a registered IP address. As the Internet grew, however, it became apparent that the number of hosts would quickly exceed the number of possible IP addresses.

One solution to the problem is Classless Interdomain Routing (CIDR). Classfull addresses are IP addresses that use the default subnet mask. They are classfull because the default subnet mask is used to identify the network and host portions of the address. Classless addresses are those that use a custom mask value to separate network and host portions of the IP address. CIDR allows for variable length subnet masking (VLSM) and enables the following features:

  • Subnetting, dividing a network address into multiple smaller subnets. For example, this allows a single Class B or Class C addresses to be divided and used by multiple organizations.
  • Supernetting, combining multiple network addresses into a single larger subnet. For example, this allows multiple Class C addresses to be combined into a single network.
  • Route aggregation (also called route summarization), where multiple routes are combined in a routing table as a single route.

CIDR routers use the following information to identify networks.

  • The beginning network address in the range
  • The number of bits used in the subnet mask

For example, the routing table represents the address as, where 21 is the number of bits in the custom subnet mask.

In addition to CIDR, the following other solutions were put into place to make efficient use of available IP addresses:

  • IP version 6. IPv6 uses 128-bit addresses instead of the 32-bit addresses used with IPv4. IPv6 is not yet used on the Internet.
  • Private addressing with address translation. With private addressing, hosts are assigned an unregistered address in a predefined range. All hosts on the private network use a single registered IP address to connect to the Internet. A special router (called a network address translation or NAT router) translates the multiple private addresses into the single registered IP address.




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