What Is Subnet Mask And Subnetting?

The subnet mask is used to determine where the network number in an IP address ends and the node number in an IP address begins. A node is anything on a network that needs an IP address to communicate (a PC, server, router, etc).
A mask used to determine what subnet an IP address belongs to. An IP address has two components, the network address and the host address. For example, consider the IP address 150.215.017.009. Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network.
A bit mask that separates the portions of an IP address significant to the network from the bits significant to the subnet. For a class C address, a standard subnet mask is 255.255.255.0, which masks the first three bytes of the address, and leaves the last byte available to identify machines on the subnet.