Classless interdomain routing

What is Classless Interdomain Routing?
Classless Interdomain Routing (CIDR) enables the aggregation of different classes of IPv4 addresses. In the original IPv4 scheme, IP addresses were set according to class, and this designation was illustrated in the values of the various octets of a given IPv4 address. When the IETF and other similar organizations began to realize that simply dividing the IP address space into classes was not enough to save IP address space, the concept of CIDR was developed.

Classless interdomain routing consists of two sections of an IPv4 address, the network section and the host section. The network part are the leftmost bits of a given IP address. As the term suggests, this identifies a given network. The rightmost bits are the host part, and as you may have guessed, these bits identify a particular host within a network. So when a network reaches a certain number of nodes, CIDR allows the host part of an IPv4 address to actually borrow bits from the network part, so the address space is preserved and more control is possible within the local network.