Subnet Calculator
Plan or interpret IPv4 subnets.
What this checks
- Base IPv4 network, subnet mask, broadcast address, total addresses, and usable host count.
- VLSM subnet allocations for each requested host group.
- Network address, first and last usable host, broadcast address, wildcard mask, and subnet mask for each allocation.
How to interpret results
- The largest host requirements are allocated first to reduce address fragmentation.
- Unused addresses show the remaining space inside the base network after all requested subnets are placed.
- If the requested subnets do not fit, choose a larger base network or reduce host requirements.
Common issues
- Host counts are interpreted as usable hosts, so normal IPv4 subnets reserve network and broadcast addresses.
- Only IPv4 VLSM planning is supported here; IPv6 subnet planning uses different conventions.
When to use the subnet calculator
Use this page when one base network has to be split into multiple child networks for offices, voice, guest access, links, or other host groups. It sorts the requested subnet sizes from largest to smallest so you can see whether the plan fits and where address space remains.
If you already know the exact CIDR block and only need its network, broadcast, and usable range, the CIDR calculator is the simpler tool. This page is designed for planning, not just expansion.
Privacy note
This subnet calculator runs entirely in the browser. The network and host requirements are not sent to the server.
Key Tools and References
Frequently Asked Questions
What is VLSM subnetting?
Variable Length Subnet Masking lets one base network be split into differently sized subnets so larger host groups get larger blocks and smaller groups waste fewer addresses.
How are host requirements allocated?
The calculator sorts requested host counts from largest to smallest, chooses the smallest power-of-two block that can fit each subnet, and allocates them inside the base CIDR range.
Does this subnet calculator send data to the server?
No. The subnet and VLSM calculations run locally in your browser.
How is this different from the CIDR calculator?
The subnet calculator is for planning or splitting a base network into multiple subnets. The CIDR calculator is better for expanding one existing prefix and reading its host range, mask, wildcard, and private/public context.