What is a subnet mask and who would use it?

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. Subnetting enables the network administrator to further divide the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is: 10010110.11010111.00010001.00001001 The Class B network part is: 10010110.11010111 and the host address is 00010001.00001001

Subnetting an IP Network is done for a various reasons. That includes organization, use of different physical media such as Ethernet, FDDI, WAN, etc., preservation of address space, and security. The biggest and most popular reason is to control network traffic within the network. In an Ethernet network, all nodes on a segment see all the packets transmitted by all the other nodes on that segment. This can lead to heavy performance loss , A router is used to connect IP networks to minimize the amount of traffic each segment must receive. http://networkguide.blogspot.com