Variable Length Subnet Mask

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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
Senior Editor
Margaret Rouse
Senior Editor

Margaret 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 IT definitions have been published by Que in an encyclopedia of technology terms and cited in articles by the New York Times, Time Magazine, USA Today, ZDNet, PC Magazine, and Discovery Magazine. She joined Techopedia in 2011. Margaret's idea of a fun day is helping IT and business professionals learn to speak each other’s highly specialized languages.