As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. One technique is called Variable-Length Subnet Masks (VLSM). With VLSM, a network administrator can use a long mask on networks with few hosts, and a short mask on subnets with many hosts.

In order to use VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated Intermediate System to Intermediate System (Integrated IS-IS), Enhanced Interior Gateway Routing Protocol (EIGRP), RIP v2, and static routing.

VLSM allows an organization to use more than one subnet mask within the same network address space. Implementing VLSM is often referred to as "subnetting a subnet", and can be used to maximize addressing efficiency.

Classful routing protocols require that a single network use the same subnet mask. Therefore, network 192.168.187.0 must use just one subnet mask such as 255.255.255.0.

VLSM is simply a feature that allows a single autonomous system to have networks with different subnet masks. If a routing protocol allows VLSM, use a 30-bit subnet mask on network connections, 255.255.255.252, a 24-bit mask for user networks, 255.255.255.0, or even a 22-bit mask, 255.255.252.0, for networks with up to 1000 users.