Full form: Variable Length Subnet Masking
Also known as: Variable Length Subnet Masking, variable-length subnetting
Quick Definition
Using different subnet mask lengths within the same major network to minimise IP waste.
VLSM allows a network engineer to divide an IP address space into subnets of different sizes, matching the number of hosts needed in each segment. For example, a WAN link between two routers only needs a /30 (2 usable hosts), while an office LAN may need a /24 (254 usable hosts). VLSM avoids wasting addresses by sizing each subnet appropriately. All modern routing protocols (OSPF, EIGRP, BGP) support VLSM.
RIPv1 does not support VLSM because it is a classful routing protocol that does not include subnet mask information in routing updates. RIPv2, OSPF, and EIGRP all support VLSM.
VLSM allows a network engineer to divide an IP address space into subnets of different sizes, matching the number of hosts needed in each segment. For example, a WAN link between two routers only needs a /30 (2 usable hosts), while an office LAN may need a /24 (254 usable hosts). VLSM avoids wasting addresses by sizing each subnet appropriately. All modern routing protocols (OSPF, EIGRP, BGP) support VLSM.
RIPv1 does not support VLSM because it is a classful routing protocol that does not include subnet mask information in routing updates. RIPv2, OSPF, and EIGRP all support VLSM.
VLSM falls under the IP Addressing domain of the 200-301 exam. Understanding it in context with related terms like cidr and flsm is essential for answering scenario-based questions correctly.