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VLSM Subnet Calculator

Plan variable-length subnets from one parent network and a list of host requirements. Largest-first allocation with masks, ranges and waste — in-browser.

4
Subnets allocated
192.168.1.0/24
Parent network
140
Addresses left over

VLSM allocation (largest requirement first)

SubnetNeededNetworkMaskHost rangeBroadcastUsableSpare
Sales60192.168.1.0/26255.255.255.192192.168.1.1 – 192.168.1.62192.168.1.63622
Engineering30192.168.1.64/27255.255.255.224192.168.1.65 – 192.168.1.94192.168.1.95300
Management10192.168.1.96/28255.255.255.240192.168.1.97 – 192.168.1.110192.168.1.111144
WAN link2192.168.1.112/30255.255.255.252192.168.1.113 – 192.168.1.114192.168.1.11520

What is the VLSM Subnet Calculator?

The VLSM (Variable Length Subnet Mask) Calculator carves one parent network into right-sized subnets from a list of host requirements.

  • Largest-first VLSM allocation with automatic power-of-two sizing
  • Per-subnet network, mask, host range, broadcast and waste report
  • Optional labels turn the output into a readable addressing plan
  • Detects and explains when the requests cannot fit the parent block
  • Shows leftover address space for future growth
  • Runs 100% in your browser — designs stay private

How to use the VLSM Subnet Calculator

  1. 1

    Enter the parent network in CIDR notation, e.g. 192.168.1.0/24.

  2. 2

    List the required host count for each subnet, one per line, optionally labelled like "Sales, 60".

  3. 3

    Review the allocation table: network, mask, usable host range, broadcast and spare addresses.

  4. 4

    Check the leftover-address stat to see how much of the parent block remains free.

  5. 5

    Click Copy plan to export the whole addressing plan as text.

About the VLSM Subnet Calculator

The VLSM (Variable Length Subnet Mask) Calculator carves one parent network into right-sized subnets from a list of host requirements. Give it 192.168.1.0/24 and departments needing 60, 30, 10 and 2 hosts, and it allocates /26, /27, /28 and /30 blocks contiguously with no overlaps.

It follows the textbook method taught in CCNA courses: sort requirements largest first, round each up to the smallest block whose usable hosts cover it, and lay the blocks out back-to-back so every subnet stays naturally aligned. Each allocation shows its network, mask, host range, broadcast and how many addresses are spare.

You can label each requirement ("Sales, 60") so the output reads like a finished addressing plan, ready to copy into documentation. Everything is calculated locally in your browser — your network design is never uploaded.

Frequently asked questions

What is VLSM and why is it used?

Variable Length Subnet Masking lets you split one network into subnets of different sizes instead of equal slices. A LAN with 60 hosts gets a /26 while a point-to-point link gets a /30, which wastes far fewer addresses than giving everything the largest required size.

Why are subnets allocated largest first?

CIDR blocks must sit on boundaries that are multiples of their own size. Placing the biggest blocks first keeps the running cursor aligned for everything that follows, so the plan packs tightly with no gaps or overlaps. Allocating small blocks first can strand space through misalignment.

How is each subnet's size chosen from the host count?

Each requirement is rounded up to the smallest power of two that still leaves room for the network and broadcast addresses. 60 hosts needs 62 usable, so a 64-address /26 fits; 30 hosts fits a /27 with exactly 30 usable; 2 hosts gets a /30.

What happens if my requirements don't fit the parent network?

The calculator stops and tells you exactly which request overflowed, how many addresses the full set of requests needs and how many the parent actually has, so you can either request a bigger block or trim the requirements.

Does this calculator use /31 subnets for point-to-point links?

No — it uses the classic /30 floor that most course material and legacy equipment expect, so a 2-host request gets a /30. If your gear supports RFC 3021 /31 links you can manually halve those allocations.

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