Variable Length Subnet Mask (VLSM)

Definition - What does Variable Length Subnet Mask (VLSM) mean?

A Variable Length Subnet Mask (VLSM) is a numerical masking sequence, or IP address subset, based on overall network requirements. A VLSM allows a network administrator to use long masks for networks with few hosts and short masks for networks with multiple hosts. A VLSM is used with a VLSM router and must have routing protocol support.

A VLSM is also known as a classless Internet Protocol (IP) address.

Techopedia explains Variable Length Subnet Mask (VLSM)

Key VLSM features include:
  • Simple network configuration
  • Network IP addressing through empty subnet filling
  • Greater efficiency than fixed-length subnet masks (FLSM)
  • Streamlined routing, where a router functions only with a VLSM sequence, versus a full IP address
VLSMs are used by a variety of network systems and services, including:
  • Intermediate System to Intermediate System Protocol (IS-IS)
  • Border Gateway Protocol (BGP)
  • Enhanced Interior Gateway Routing Protocol (EIGRP)
  • Cisco's Open Shortest Path First (OSPF)
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