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Classless interdomain routing (CIDR) allows for the aggregation of different classes of IPv4 addresses. In the original IPv4 scheme, IP addresses were designated according to class, and this designation was illustrated in the values of the different octets of a given IPv4 address. When the IETF and other similar organizations began to recognize that simply dividing IP address space into classes would not be sufficient enough to conserve IP address space, the concept of CIDR was developed.
Classless interdomain routing involves two portions of an IPv4 address, the network portion and the host portion. The network portion are is the left-most bits of a given IP address. As the term indicates, this identifies a given network. The right-most bits are the host portion, and as you may have guessed, these bits identify a specific host within a network. So, when a network achieves a certain number of nodes, CIDR allows for the host portion of an IPv4 address to, in effect, borrow bits from the network portion, thus allowing for the conservation of address space, and allowing for more control within the local network.