VLSM / Subnet Divider
Enter a base network and list your subnet requirements — label each one and set the host count. The calculator sorts by size (largest first), then allocates the smallest prefix that fits, placing subnets contiguously and flagging any that overflow. Or switch to Equal Split mode to divide the base into N identical subnets.
Requirements are sorted largest-first before allocation. Minimum 1 host per subnet.
| Label | Network | CIDR | Mask | First host | Last host | Usable hosts | Broadcast |
|---|
Click the copy icon on any row to copy that subnet's details to the clipboard.
Use — Guide
How to use this tool
- Enter the base network address and choose a prefix length — for example, 10.0.0.0 /24. The "Available addresses" counter updates to show the total address space.
- In "Subnet Requirements" mode, each row takes a label (e.g. "Finance") and the number of hosts that subnet needs. Add or remove rows with the "+ Add subnet" and delete buttons.
- Click "Calculate subnets" — the tool sorts your requirements largest-first and allocates the smallest prefix that fits each one, placing subnets contiguously within the base network.
- Read the result table: each row shows the subnet label, network address, CIDR prefix, subnet mask, first and last usable hosts, usable host count and broadcast address.
- If any row is flagged "Overflow", the base network is too small — either use a shorter base prefix (more addresses) or reduce host counts, then recalculate.
- Switch to "Equal Split" mode to divide the base network into N identically sized subnets instead — enter the number of subnets and the calculator shows the resulting host count per subnet.
FAQ — Questions
Frequently asked questions
01What is VLSM and why does it matter?
VLSM (Variable Length Subnet Masking) lets you use different prefix lengths within a single address block — rather than splitting a network into equal-sized pieces, you give each subnet exactly the address space it needs. A subnet serving 5 devices can be a /29 (6 usable hosts) while one serving 100 devices gets a /25 (126 usable hosts), all within the same base network. VLSM wastes far fewer addresses than fixed-length subnetting and is standard in modern network design.
02Why does the calculator allocate the largest subnet first?
Allocating the largest subnet first is the standard VLSM best practice. Because subnets must be aligned to their own prefix boundary, placing larger blocks first — before smaller ones consume the aligned address space — minimises fragmentation and wasted addresses. This calculator sorts your requirements by descending host count before allocating, just as a network engineer would.
03Is my data kept private?
Yes. All VLSM calculations run entirely in your browser using JavaScript. No network addresses, host counts or subnet details are ever sent to Peritus Digital or any third party.
04Can Peritus Digital help design our network addressing scheme?
Absolutely. VLSM planning is straightforward for small networks, but grows complex quickly when you factor in VLANs, firewall zones, future growth and multi-site routing. Peritus Digital's Newcastle team provides managed IT and full network design services for Hunter Region businesses — from IP addressing and switch configuration through to SD-WAN and structured cabling. Contact us to discuss your requirements.
More — Keep exploring
Related free tools
Want hands-on help, not just a check? Explore ourManaged IT & Support service.
Need help with your network design?
From VLSM planning to full network deployment — Peritus Digital can help.
Our Newcastle team designs and manages networks for Hunter Region businesses — VLANs, firewall rules, SD-WAN, structured cabling and everything in between. If you're sizing subnets, chances are you're planning infrastructure. Let's talk.