IP Address and Netmask:

Can calcutate subnet mask.

A subnetwork or subnet is a logical subdivision of an IP network. The practice of dividing a network into two or more networks is called subnetting.

Computers that belong to a subnet are addressed with a common, identical, most-significant bit-group in their IP address. This results in the logical division of an IP address into two fields, a network or routing prefix and the "rest" field or host identifier. The rest field is an identifier for a specific host or network interface.