Why are my subnets not in the order I typed them?

They are — the table preserves your order. The allocation is done largest-first, because that packs the space without leaving unusable fragments, but only the placement order differs.

Why did a two-host link get a /31?

RFC 3021 defines a /31 as a point-to-point link with two usable addresses and no broadcast. Using a /30 there wastes half the block. If your equipment does not support /31 links, set a fixed /30 on that row.

What does the growth allowance do?

It multiplies each host count before the prefix is chosen, so 100 hosts at 20% is sized for 120. That often pushes a segment into the next prefix, which is exactly the decision worth making deliberately rather than discovering later.

What happens if a segment does not fit?

It is listed separately with the reason, and the rest of the plan still allocates. It is never silently dropped or given a smaller subnet than it asked for.

Is my addressing plan uploaded?

No. Everything is calculated in your browser and the exports are generated there too. Nothing about your network leaves your machine.

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VLSM Designer

Give it a parent network and what each segment needs to hold. It works out the subnets, packs them without gaps, and tells you when they do not fit.

Local toolYour network plan stays in your browser. Nothing you enter is uploaded, logged, or sent to any server.

Parent network

Growth allowance

Segments

How the allocation works

Variable Length Subnet Masking means giving each segment only as much address space as it needs, instead of carving the parent into equal pieces. A link needing two addresses gets a /31; a floor of 500 machines gets a /23.

Largest first. Segments are allocated in descending order of size, which is what keeps the result compact and free of unusable gaps. Allocating smallest-first strands fragments too small to use. The table still shows your segments in the order you typed them — only the placement order differs.

Prefix choice. For IPv4 a segment needing n hosts gets the smallest prefix where 2^{host bits} − 2 ≥ n, because the network and broadcast addresses cannot be assigned. Two exceptions: a segment needing two addresses gets a /31, which RFC 3021 defines as a point-to-point link with no broadcast, and one needing a single address gets a /32.

Growth is applied before sizing, so 100 hosts with 20% headroom is sized for 120 and lands on a /25. This frequently changes the prefix, which is the point of asking.

Anything that does not fit is listed separately with the reason. It is never silently dropped or quietly given a smaller subnet than it asked for.

Related tools

Subnet Calculator inspects a single prefix in detail. CIDR Aggregator merges a finished plan down to the fewest prefixes. IP Range to CIDR converts a range you were handed into blocks you can allocate from.

VLSM Designer

Segments

Address space

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