Fixed Length Subnet Masking (FLSM) is a subnetting technique where a block of IP addresses is divided into subnets, each having an equal number of addresses. This results in subnets with the same size and the same subnet mask. FLSM is also known as traditional subnetting and is associated with classful subnetting. It is relatively easy to configure and manage but can lead to wastage of IP addresses, as all subnets use the same subnet mask and have an equal number of hosts.