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)
Classless Internet Protocol (IP) Address