VLSM

networking

Stands for: Variable-Length Subnet Masking

Using different subnet sizes within the same network, sized to each segment's need.

Variable-Length Subnet Masking abandons one-size-fits-all masks: a point-to-point link gets a tiny /30 while a user LAN gets a roomy /24, so addresses are not wasted. It depends on classless routing to carry the varied prefixes.

VLSM means subnetting a network with masks of different lengths, so each subnet is sized to what it actually holds. Before it, every subnet in a network shared one mask, which forced the same allocation on a point-to-point link with two hosts and a floor with two hundred.

The waste was enormous and it compounded. A link needing two addresses consumed a block sized for the largest subnet in the plan, and across a large network that arithmetic ran into thousands of unusable addresses. Variable-length masks let a point-to-point link take a two-address block while a user floor takes what it needs, which is most of why IPv4 lasted as long as it did.

The design discipline it demands is allocation order. Assign the largest subnets first from the top of the range and work downward, because filling small blocks in first fragments the space and leaves no contiguous room for the large ones. That same ordering makes summarization possible later, since aggregating routes requires the blocks beneath a summary to be contiguous, and a network subnetted in whatever order requests arrived usually cannot be summarized at all.

Also known as: vlsm

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