A variable-length subnet mask (VLSM) is a sequence of numbers of variable 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 VLSM 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 a subnet mask 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. The VLSM facilitates efficient use of IP address space in networks whose subnets are not filled to capacity. In networks having many unassigned IP addresses, a VLSM can provide more efficient use of address space than the more common fixed-length subnet mask (FLSM) in which the sequence of numbers always has the same length. Variable-length subnet masking also makes it easier for system administrators to change the configuration of a network as an organization evolves.