VLSM

Variable Length Subnet Masking (VLSM) is a subnetting technique used in IPv4 networks that allows subnets to be created with different subnet masks within the same network, rather than using a single fixed mask for all subnets. This enables more efficient use of the available address space by tailoring the size of each subnet to its actual host requirements.

VLSM is a key practice enabled by CIDR (classless inter-domain routing). With VLSM, a larger network is subdivided into multiple subnets of varying sizes, applying masks such as /26, /27, /28, and so on. Routers must correctly interpret these varying masks, and routing tables must support classless routing to prevent conflicts and ensure proper delivery of packets.

The primary benefits of VLSM are improved address utilization and greater flexibility in network design. It allows different departments or segments to receive appropriately sized subnets, supports hierarchical addressing, and can enable route aggregation to reduce routing table size when combined with proper planning. The main trade-off is increased planning complexity and potentially larger routing tables, since more subnets with distinct masks must be managed.

Example: in a 192.168.0.0/24 network, you might allocate subnets to fit exact needs as follows: 192.168.0.0/26 for 60 hosts, 192.168.0.64/28 for 10 hosts, 192.168.0.80/27 for 20 hosts, and 192.168.0.112/25 for 100 hosts. This leaves some addresses unused (240–255) but minimizes wasted space inside the overall 192.168.0.0/24 block.

Commonly used with routing protocols that support CIDR, such as OSPF, EIGRP, and BGP, VLSM requires careful address planning and consistent subnetting practices.