Class A addresses are long gone,
and the world is quickly running out of Class B addresses.
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Many Class C addresses are still available, but the small address space of a
Class C network (254 hosts maximum) is a severe limitation in the high-volume game of
Internet service providers (ISPs).
It is possible to assign a range of Class C networks
to a network owner who needs more than 254 addresses.
However, treating multiple Class C networks as separate entities when they are
all going to the same place only clutters up routing tables unnecessarily.
Classless Internet Domain Routing (CIDR) is a
technique that allows a block of network IDs to be treated as a single entity
in routing tables. CIDR groups a range of network IDs into a single address entry
using what is called a supernet mask.
You can think of a supernet mask as something like the opposite of a subnet mask.
Instead of designating additional bits for identifying the network,
the supernet mask in effect takes bits away from the network ID.
The addresses in the range are therefore identified by the network address bits
that the networks in the range hold in common.
