The term variable-length subnet mask (VLSM) refers to the fact that a network can configure with different subnet mask. The basic idea behind VLSM is to offer more flexibility by dividing a network into multiple subnets while optimizing the allocation of varying amounts of host space between subnets. Without VLSM, only one subnet mask can apply to an entire network. It would restrict the number of hosts given the number of subnets required. If you select the subnet mask so that it has enough subnets, you might not be able to assign enough host numbers in each subnet. The same is true for hosts; A mask that allows enough hosts may not provide enough subnet space. VLSM provides the ability to assign subnets with varying numbers of hosts, allowing the network administrator to use the address space better.