VLSM Calculator
Enter one or more base networks and a list of named subnets, with the number of hosts each one needs. You'll get back a right-sized, non-overlapping block for each subnet.
Split one or more base networks into right-sized subnets for each department or purpose. Larger requirements are packed first so every block stays properly aligned — no gaps, no overlaps. With several base networks, subnets fill the first one, then carry on into the next.
How it works
Each named requirement is first matched to the smallest subnet size that can hold it. A 50-host requirement needs a /26 (62 usable hosts). A 10-host requirement only needs a /28 (14 usable hosts).
Those subnets are then packed into the base network, largest first. Every subnet size is a power of two, so allocating in this order automatically lines up each block on a proper boundary. This is the same approach you'd use when subnetting by hand.
FAQ
- What is VLSM?
- VLSM stands for Variable Length Subnet Mask. It means splitting one network into subnets of different sizes, based on how many hosts each one actually needs. That's instead of dividing it into equal-sized chunks, which wastes addresses on the smaller groups.
- How does the tool decide the order to allocate subnets?
- It always allocates the largest requirement first. That's not just a display choice. Packing from largest to smallest keeps every block aligned on its own boundary, so there are never any gaps or overlaps between subnets.
- What happens if the subnets don't fit in the base network?
- You'll get a clear error naming which requirement didn't fit. Then you can pick a larger base network, add another base network, or reduce the number or size of your subnets.
- Can I use more than one base network?
- Yes. Click "Add base network" to list several blocks, for example 192.168.1.0/24 and 10.0.0.0/24. Subnets are still allocated largest first; each one goes into the first base network (in your order) that has room left. The usage bars show how much of each block is used.
- Can I save or share the plan?
- Yes. "Copy link" gives a link that reopens the same plan, "Download CSV" saves it for Excel or Google Sheets, and "Print / Save as PDF" makes a printable copy.