Answers:
Identifies which part of an IP address is reserved for the network, and which part is available for host use.
An IP address has two components, the network address and the host address. A subnet mask separates the IP address into the network and host addresses (<network><host>). Sub netting further divides the host part of an IP address into a subnet and host address (<network><subnet><host>) if additional sub network is needed. It is called a subnet mask because it is used to identify network address of an IP address by performing a bitwise AND operation on the net mask.