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Networking Fundamentals · Intermediate

Variable Length Subnet Mask (VLSM)

Using different subnet mask lengths within the same address space to match each subnet's size to its host requirement.

Definition

VLSM avoids wasting addresses by allocating large subnets to large LANs and small subnets, such as /30 or /31, to point-to-point links. Assign the largest subnets first to keep allocations aligned and non-overlapping. VLSM requires classless routing protocols that carry the prefix length.

Example

From 10.0.0.0/24, allocate a /25 for 100 users, a /26 for 50 users, and /30s for WAN links.

Exam tip

Allocate largest to smallest, and verify no subnets overlap.

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