What is the Subnet Calculator?
Subnetting divides one block of IP addresses into smaller networks, for example a separate subnet for each office floor, VLAN, cloud tier or customer. Getting the boundaries right by hand is fiddly: every subnet must start on a multiple of its size, and the network and broadcast addresses are not usable.
Enter the network you have, then either ask for a number of equal subnets, ask for subnets big enough for a number of hosts, or list each subnet you need with its host count for a variable-length (VLSM) plan. You get every subnet’s range, mask, broadcast address and usable hosts, a picture of how the space is used, and a CSV you can paste into a spreadsheet or IPAM tool.
How to use it
- Enter the network to divide, like 10.0.0.0/16 or 192.168.10.0/24.
- Choose Equal subnets, Hosts per subnet or VLSM.
- Enter the number of subnets, the hosts each needs, or your list of named subnets.
- Review the plan and copy or download it as CSV.
Example
192.168.10.0/24 · VLSM: Servers 100, Office 20, Link 2
Servers 192.168.10.0/25 (126 hosts) Office 192.168.10.128/27 (30 hosts) Link 192.168.10.160/30 (2 hosts)
Features
- Three ways to split. A number of equal subnets, subnets sized for N hosts, or a VLSM plan of different sizes.
- VLSM done right. Largest subnets placed first on correct boundaries, so space is not wasted.
- Full details. Network, mask, first and last host, broadcast and usable hosts for every subnet.
- Space at a glance. A bar shows how much of the network each subnet uses and what is left free.
- CSV export. Copy or download the plan for Excel, Google Sheets or your IPAM tool.
- Private. Calculated in your browser; your network plan is not uploaded.
Frequently asked questions
What is VLSM?
Variable Length Subnet Masking lets subnets in the same network have different sizes. Instead of making every subnet as big as the largest one, each gets the smallest block that fits its hosts, which saves addresses.
How many subnets can I make from a /24?
Borrowing 1 bit gives 2 × /25, 2 bits give 4 × /26, 3 bits 8 × /27 and so on, up to 64 × /30 for point-to-point links (2 hosts each). Each extra bit doubles the number of subnets and halves their size.
Why does a subnet for 50 hosts get 62?
Subnets come in powers of two. 50 hosts plus the network and broadcast addresses need 52 addresses, so the next size up, 64 (/26), is used, giving 62 usable hosts.
Should I leave room to grow?
Yes. Plan for growth by asking for more hosts than you need today, or leave free space between subnets so each can be enlarged later.
Last updated · Free, no sign-up, runs in your browser.