Subnetsplit
Help Centre
Subnetsplit is a CIDR subnet calculator that runs entirely in your browser. You enter a parent CIDR block and the subnets you need — by host count or prefix length — and it produces a verified, non-overlapping allocation plan.
No account, no sign-up, no data leaves your machine.
How to use the tool
- 1. Open the App page.
- 2. Enter the parent CIDR block (e.g.
10.0.0.0/16). - 3. Add subnet requests by clicking + Add subnet. Each row needs a name and a size — enter a host count (e.g.
500) or a prefix length (e.g./24). - 4. Click Allocate. The tool verifies every subnet against every other one and against the parent bounds, then renders the plan.
- 5. Review the results: network address, broadcast address, usable host count, dotted-decimal mask, and address range for each subnet. Any leftover gaps are listed separately.
- 6. Use Copy plan to paste the table into a ticket or config, or Download CSV for a spreadsheet-friendly export.
Top questions about Subnetsplit
Does Subnetsplit work offline?
Yes. The entire tool is a single-page JavaScript application. Once the page has loaded, no further network requests are made. The computation, overlap verification, CSV generation, and clipboard copy all run locally. If you need the tool without internet access after the initial page load, it will continue to work.
How do I specify subnet sizes — by host count or prefix length?
Both are supported. Enter a bare number (e.g. 500) and Subnetsplit calculates the smallest prefix that fits the required hosts. Enter a slash-prefix (e.g. /24) and it uses that exact prefix length. When using host counts, the tool deducts the network and broadcast addresses per RFC 3021, except for /31 (point-to-point, 2 usable) and /32 (loopback, 1 usable).
What does the overlap check actually verify?
Every requested subnet is checked against every other requested subnet and against the parent block before the plan renders. The tool converts each CIDR to a 32-bit integer range and verifies that no two ranges intersect. If an overlap is detected, the offending row is flagged in red and the plan is not shown until the conflict is resolved. Gaps in the allocation are also named so you can see what address space remains available.
How do I save my allocation plan?
Subnetsplit runs entirely in your browser and does not store plans on a server. To save a plan, use the Copy plan button to copy the formatted text table to your clipboard and paste it into a document, ticket, or config. Use the Download CSV button to download a CSV file for spreadsheets. Both exports include the allocation details, overflow warnings, and remaining address space. Plans are lost when you close or refresh the tab, so export before navigating away.
What is included in the Free tier?
Everything. Subnetsplit is completely free — unlimited calculations, overlap verification, CSV export, and clipboard copy. No account, no sign-up, no data leaves your browser. There are no paid tiers and no subscriptions. The full product is available to everyone at no cost.
Does Subnetsplit support IPv6?
No. Subnetsplit currently handles IPv4 CIDR notation only. IPv6 subnetting is a separate feature that would require significant additional arithmetic and testing. There is no timeline for adding IPv6 support at this stage.
I entered a /31 and it shows 2 usable hosts — is that correct?
Yes. Per RFC 3021, a /31 point-to-point link does not require a network or broadcast address, so both addresses are usable as host addresses. Similarly, /32 is treated as a single-host loopback with 1 usable address. All other prefixes follow the standard formula: usable = 2^(32-prefix) - 2.
What Subnetsplit does not do yet
Subnetsplit is a client-side calculator. The following features are either not built or deliberately outside scope:
- No server-side storage — Plans are not saved. Export before closing the tab.
- No IPv6 support — IPv4 CIDR only.
- No API — The tool cannot be called programmatically.
- No team sharing or workspaces — Every session is single-user.
- No CSV import — Subnet requests must be entered manually.
- No saved plans — The tool is ephemeral by design. Export before closing the tab.
- No network reach — The tool does not query or configure any device. It is a calculator only.
- No paid tiers — Subnetsplit is completely free with no subscriptions.
How to reach us
Subnetsplit is a product of Agen2ic, LLC. For questions, feature requests, bug reports, or anything else — email us.
Support email
subnetsplit-p1@agen2ic.ai
We aim to respond within two business days.
Before emailing, check the FAQ above — most questions are answered there. If you are reporting a bug, please include the parent CIDR block, the subnet sizes you entered, and what you expected vs what you saw.