Classless Inter-Domain Routing (CIDR) is a method for allocating IP addresses and for IP routing. CIDR is a notation for describing blocks of IPv4 addresses and is used heavily in various networking configurations. IPv4 addresses contain 4 octets, each composed of 8 bits, giving values between 0 and 255. The decimal value after the slash is the number of bits consisting of the routing prefix. This in turn can be translated into a netmask, and also designates how many available addresses are in the block. That is, the IPv4 address portion of the output CIDR block must not contain any 1 (in binary notation) set bits in the host part of the address.
For example: 201.105.7.34/24 represents the IPv4 address 201.105.7.34 and its associated routing prefix 201.105.7.0, or equivalently, its subnet mask 255.255.255.0, which has 24 most significant bits of 1.
It is also intended to be a teaching tool; useful for both students and teachers.