NetDesignKit

Free tool · IPv4

Subnet and VLSM planner

Enter an address block and the hosts each segment needs. The planner allocates the largest subnets first so every subnet stays on its natural boundary.

Requirements

1,024 addresses.

SegmentHosts

The example is a typical branch site. Replace it with your own segments.

Allocation

Block
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Allocated
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Free
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Used
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SegmentHosts neededSubnetUsable rangeBroadcastMaskWildcardUsableSpare
Need a full IP plan, VLAN register and port map? The LLD and IP plan pack is coming soon.See templates

How the planner works

  1. For each segment it finds the smallest subnet whose usable hosts cover the need. Usable hosts are the subnet size minus two, for the network and broadcast addresses.
  2. It sorts segments from largest to smallest and allocates them in order from the start of the block. This is standard VLSM practice: a large subnet placed first always starts on a boundary its size divides into.
  3. Any segment that no longer fits is flagged with the prefix it would need, so you can enlarge the block or trim requirements.

Worked example

A branch site has 10.20.0.0/22 (1,024 addresses) and needs seven segments:

Corporate users 250 hosts → /24 10.20.0.0/24 254 usable Voice 200 hosts → /24 10.20.1.0/24 254 usable Wireless staff 120 hosts → /25 10.20.2.0/25 126 usable Wireless guest 100 hosts → /25 10.20.2.128/25 126 usable Printers 30 hosts → /27 10.20.3.0/27 30 usable Management 20 hosts → /27 10.20.3.32/27 30 usable WAN link 2 hosts → /30 10.20.3.64/30 2 usable

That uses 836 of 1,024 addresses (82%) and leaves 10.20.3.68 to 10.20.3.255 free for growth.

Frequently asked questions

Why does a segment needing 250 hosts get a /24?

A /25 has only 126 usable addresses. The next size up, a /24, has 254, which covers 250 with 4 spare.

Should I size for today or for growth?

Enter the host count you want to support, including growth. Many designs add 20 to 50% headroom for user and wireless segments.

Does this account for gateway and HA addresses?

No. If your design uses a gateway plus two HA or VRRP addresses, add three to the host count for that segment.

Why is /31 not offered for point-to-point links?

The planner uses /30 as the minimum because every platform supports it. Many routers also support /31 links (RFC 3021); you can split a /30 by hand if your standard allows it.

Is my data stored?

No. The planner runs entirely in your browser. Nothing you type is sent to a server.

Planning aid only. Check results against your IPAM and design standards before deployment.

Free download

Subnetting cheat sheet

Two A4 pages: every prefix from /8 to /32 with masks, wildcards and host counts, private and special ranges, a four-step subnetting method, VLSM, summarisation and an IPv6 quick reference. See what’s inside.