Variable Length Subnet Mask

What Does Variable Length Subnet Mask 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.

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

Techopedia Explains Variable Length Subnet Mask

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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Margaret Rouse

Margaret Rouse is an award-winning technical writer and teacher known for her ability to explain complex technical subjects to a non-technical, business audience. Over the past twenty years her explanations have appeared on TechTarget websites and she's been cited as an authority in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine and Discovery Magazine.Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages. If you have a suggestion for a new definition or how to improve a technical explanation, please email Margaret or contact her…