When addressing subnets, a distinction is made between subnet masks with variable address length, the Variable Length Subnet Mask( VLSM), and with fixed address length, the Fixed Length Subnet Mask (FLSM).
For subnets with a fixed subnet mask, this is used for all subnets in the network, regardless of the number of hosts that are on each subnet. It is a fixed-length string of binary digits that is used to control the routing of data packets to individual subnets. Incoming data packets are examined for the subnet number and routed to the appropriate data station or host via the routers. The routers filter only the digits of the subnet mask from the IP address, thereby reducing the administrative overhead of the routers.
Since fixed-length subnet masking does not use the entire subnet address space, the method is not sufficiently efficient. Therefore, the variable-length address method, variable-length subnet mask (VLSM), was developed, in which the subnet address space is adapted to the required subnet addresses. This method is an improvement on the original fixed-length subnet mask.