A: Although it is formatted like an IP address, a subnet mask is actually a filter that informs a TCP/IP system how to read an IP address. All 32-bit IP addresses are composed of a host ID, that identifies a network interface in a particular machine, and a network ID, that identifies the network on which that machine is located. Since the lengths of these IDs can vary, the system needs to be told where the dividing line is between the two.

When expressed in binary form, a subnet mask consists of 32 ones and zeroes. When compared with the 32 bits of an IP address, a one in the subnet mask indicates that a bit is part of the network ID, and a zero that it is part of the host ID. Thus, when a subnet mask of 255.255.255.0 is applied to the IP address 192.168.99.100, the mask translates into a binary value of 11111111.11111111.11111111.00000000, meaning that the first 24 bits of the IP address (192.168.99) identify the network, and the final 8 bits (100) identify the host.

For more information, see page 42 of TCP/IP Administration.