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(also variable length subnet mask)

VLSM definition

A VLSM (short for “variable length subnet mask”) is a computer networking technique to divide an IP network into subnets with different subnet masks. VLSM allows network designers to give each subnet a different number of IP addresses, ultimately resulting in less network congestion and wasted IPs.

Longer VLSMs can support more hosts and have more IP addresses. This practice stands in contrast to traditional fixed-length subnet masks (FLSM), which use the same subnet mask for all subnets.

See also: subnet, IPv4, class A IP address, netmask

Common VLSM routing protocols

  • Open Shortest Path First (OSPF): An interior gateway protocol widely used for routing within autonomous systems. OSPF can take into account varying subnet mask lengths when calculating the shortest path to the destination.
  • Border Gateway Protocol (BGP): An exterior gateway protocol used for routing between different autonomous systems, typically internet service providers and large enterprise networks.
  • Routing Information Protocol (RIP): An older distance-vector routing protocol that supports VLSM. RIP can exchange routing information with neighboring routers, including the subnet mask information.
  • Enhanced Interior Gateway Routing Protocol (EIGRP): A proprietary Cisco routing protocol with advanced features (such as fast convergence and load balancing). EIGRP can advertise routes with different subnet mask lengths and efficiently converge in networks with VLSM.