Subnetsplit
Carve a CIDR block into subnets. Verified before you see the plan.
Enter the parent block and the sizes you need — by host count or prefix length. The allocation plan comes back with ranges, masks, usable hosts, broadcast addresses, and every row checked against every other row before it renders.
Every allocation checked against every other before it renders.
Subnetting is arithmetic nobody enjoys doing twice. The failure is never the math itself — it is an overlap nobody noticed, a block allocated twice, or a /26 written down where a /24 was meant. Those surface weeks later as an address conflict in an environment somebody is depending on.
Subnetsplit verifies every requested subnet against every other one before it shows the plan. Overlaps are flagged, gaps are named, and the whole allocation is cross-checked so you ship the configuration with the right numbers the first time.
Three inputs, one verified plan.
- Parent block Enter the CIDR notation for the address range you want to divide — e.g. 10.0.0.0/16 or 192.168.0.0/24.
- Subnet requests Add each subnet by name and size. Size can be a host count (500) or a prefix length (/24). Add as many as you need — rows are checked individually before allocation.
- Allocation plan Subnetsplit packs them largest-first, computes every address range, and returns a table with network, broadcast, usable hosts, dotted-decimal mask, and any leftover gaps.
Free. No account, no sign-up, no data leaves your browser.
Subnetsplit runs entirely in the browser. There is no server, no upload, no account to create, and nothing is stored. Everything you type stays on your machine. The same applies to every tier — the free tier is the full product.
No network reach, no device config, no persistence.
Subnetsplit is a calculator — it computes and verifies subnet allocations. It does not reach any network, configure any device, look anything up over the wire, or store a plan after the tab closes. There is no API, no cloud sync, no team sharing, and no scheduled export. If you need to save a plan, download the CSV or copy the text table before you close the tab.
The arithmetic is the whole product.
Every allocation is computed from first principles: a parent CIDR is parsed to a 32-bit integer, each request is converted to the smallest prefix that satisfies the host count, subnets are sorted by size descending and packed sequentially, and the result is checked against the parent bounds. The code is one page of deterministic JavaScript — there is no hidden state, no rounding path, and no dependency on an external service.
Host-count assignments follow RFC 3021 for /31 point-to-point links and treat /32 as a single-host loopback. The formulas are documented inline and verifiable against any subnetting reference.
One click to copy or download. No retyping.
The allocation plan leaves your browser as a formatted text table for pasting into a ticket or a config, and as a CSV file for spreadsheets and documentation. Both exports include overflow information and the remaining address space.
Enter your CIDR block, get a verified plan. No account needed.
Everything runs in your browser. Nothing is uploaded.