Variable Length Subnet Mask (VLSM)

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)