How to use the Subnet Splitter (VLSM) tool
- Enter the network to split, for example 10.10.0.0/16.
- Choose: a number of equal subnets, a number of hosts per subnet, or different sizes (VLSM).
- Click “Split” and review the table.
- Copy the plan as CSV for your documentation or spreadsheet.
Equal subnets
Splitting a network into equal parts borrows bits from the host part. Each borrowed bit doubles the number of subnets and halves their size. Splitting a /24 into 4 subnets borrows 2 bits and gives four /26 networks with 62 usable hosts each. If you ask for a number that is not a power of two, such as 5, the calculator rounds up to the next power of two (8), because subnets must align on bit boundaries.
VLSM: different sizes
Real networks rarely need equal subnets. An office might need 100 addresses for staff, 50 for Wi-Fi guests, 20 for printers and phones, and 2 for a router link. Variable Length Subnet Masking gives each segment the smallest subnet that fits.
The calculator allocates the largest requirement first, which avoids fragmentation, and aligns every subnet to its own size. For the example above inside 192.168.0.0/24, you get a /25 (126 hosts), a /26 (62), a /27 (30) and a /30 (2), still leaving room for growth.
Planning tips from practice
- Leave spare space in each subnet. Device counts always grow.
- Keep related subnets together so they can be summarised into one route later.
- Document the plan right away: the CSV export goes straight into a spreadsheet or IPAM tool.
- Use /30 or /31 for router-to-router links and do not waste a /24 on them.
Privacy
All calculations run in your browser. Nothing you enter is sent anywhere.
Frequently asked questions
Why do I get 8 subnets when I asked for 5?
Subnets are created by borrowing whole bits, so the count is always a power of two. Use VLSM mode if you need exactly five subnets of different sizes.
What order are VLSM subnets allocated in?
Largest first, which is the standard method and keeps the address space contiguous. The table lists them in the order you entered them.
Is there a limit?
Up to 4,096 subnets per split, which is plenty for planning and keeps the page fast.
Can I split IPv6 networks?
Use the IPv6 Subnet Calculator, which lists the subnets of a chosen prefix length.