Classless Interdomain Routing (CIDR)

Classless Interdomain Routing (CIDR)

1.     CIDR was invented several years ago to keep the Internet from running out of IP addresses.

2.     The class system of allocating IP addresses can be very wasteful. Anyone who could reasonably show a need for more than 254 host addresses was given a Class B address block of 65,533 host addresses.

3.     Even more wasteful was allocating companies and organizations Class A address blocks, which contain over 16 million host addresses.

4.     People realized that addresses could be conserved if the class system was eliminated. By accurately allocating only the amount of address space that was actually needed, the address space crisis could be avoided for many years.

5.     This solution was first proposed in 1992 as a scheme called supernetting. Under supernetting, the class subnet masks are extended so that a network address and subnet mask could, for example, specify multiple Class C subnets with one address.

6.     For example, if you needed about a thousand addresses, you could supernet 4 Class C networks together.

7.     CIDR will probably keep the Internet happily in IP addresses for the next few years at least.

8.     After that, IPv6, with 128 bit addresses, will be needed. Under IPv6,0 even careless address allocation would comfortably enable a billion unique IP  addresses for every person on earth