A fixed-length subnet mask (FLSM) is a sequence of numbers of unchanging length that streamlines packet routing within the subnets of a proprietary network. A subnet can be a geographically defined local area network (LAN). Alternatively, a subnet may define security boundaries, departmental boundaries, multicast zones or hardware security parameters.
Once a packet has arrived at an organization's main gateway with its network number, that packet is routed to its ultimate destination using a subnet number. The FLSM is usually a string of binary digits shown over the subnet number, telling the router which parts of the subnet number to look at. A binary "1" over a particular digit in the subnet number says "Pay attention to this digit." A "0" says "Ignore this digit."
The use of an FLSM saves a router the task of having to handle an entire IP address because the router deals only with the digits selected by the mask. Nevertheless, this method of subnet masking can result in inefficient use of IP address space because subnets are rarely filled to capacity. In networks having many unassigned IP addresses, a variable-length subnet mask (VLSM) can provide more efficient use of address space.