CIDR (Classless inter-domain routing)

CIDR (Classless inter-domain routing) is a method of public IP address assignment. It was introduced in 1993 by Internet Engineering Task Force with the following goals:

  • to deal with the IPv4 address exhaustion problem
  • to slow down the growth of routing tables on Internet routers

Before CIDR, public IP addresses were assigned based on the class boundaries:

  • Class A – the classful subnet mask is /8. The number of possible IP addresses is 16,777,216 (2 to the power of 24).
  • Class B – the classful subnet mask is /16. The number of addresses is 65,536
  • Class C – the classful subnet mask is /24. Only 256 addresses are available.

Some organizations were known to have gotten an entire Class A public IP address (for example, IBM got all the addresses in the range). Since these addresses can’t be assigned to other companies, there was a shortage of available IPv4 addresses. Also, since IBM probably didn’t need more than 16 million IP addresses, a lot of addresses were unused.

To combat this, the classful network scheme of allocating the IP address was abandoned. The new system was classsless – a classful network was split into multiple smaller networks. For example, if a company needs 12 public IP addresses, it would get something like this:

The number of usable IP addresses can be calculated with the following formula:

2 to the power of host bits – 2

In the example above, the company got 14 usable IP addresses from the – range because there are 4 host bits and 2 to the power of 4 minus 2 is 14 The first and the last address are the network address and the broadcast address, respectively. All other addresses inside the range could be assigned to Internet hosts.

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