HELP CENTRE

How to use Subnetsplit, answers to common questions, and how to reach us.

Subnetsplit is a pure client-side IPv4 subnet calculator. Everything runs in your browser — no data is uploaded, stored, or shared. This page covers the essentials.

01 / GETTING STARTED

Three steps to your first allocation plan.

Go to the calculator

Open the calculator page. You will see a form with two fields: "Parent CIDR Block" and "Subnet Requirements". No account or sign-in is needed — the page is ready to use as soon as it loads.
Enter your parent block and requirements

Type the network you want to divide into the first field — for example 10.0.0.0/16 or 192.168.1.0/24. In the second field, list the subnets you need, one per line. You can enter them by prefix length (/24), by host count (254 hosts or just 254), or as a bare number. Bare numbers up to 32 are treated as prefix lengths.
Click Calculate and review the plan

Subnetsplit packs your subnets in order of size (largest first), checks every allocation against every other one for overlap, and reports any requests that do not fit. The result table shows each subnet's CIDR, network address, broadcast address, subnet mask, usable addresses, and the full address range. Remaining free space is listed below the table.

If a subnet request exceeds the available space, it is flagged in red with the overflow highlighted. You can adjust your requirements and recalculate — each calculation starts fresh.

02 / FREQUENTLY ASKED QUESTIONS

Does Subnetsplit store my IP allocation plan?

No. Subnetsplit runs entirely in your browser. The plan you see lives only in the browser tab you opened. Closing the tab erases it. There is no server-side storage, no database, no API that receives your data. If you need to keep the plan, export it using the "Copy as Text" or "Download CSV" buttons before closing the tab.

Does Subnetsplit support IPv6?

No. The arithmetic works on 32-bit IPv4 addresses only. IPv6 subnetting follows fundamentally different rules (128-bit addresses, different prefix conventions, different broadcast behaviour) and is not supported. If you need IPv6 subnetting, Subnetsplit is not the right tool.

Can I use Subnetsplit offline or in an air-gapped environment?

Yes — after the initial page load. The page loads Google Fonts (Barlow and Fira Code) from Google Fonts the first time you visit, but once the HTML, CSS, and JavaScript are cached, the tool works fully offline. The CIDR arithmetic library is 420 lines of pure JavaScript with zero external dependencies. There is no network polling, no analytics beacon, and no form submission.

What does the "overflow" warning mean?

An overflow means one or more of your subnet requests cannot fit inside the parent CIDR block you specified. Subnetsplit allocates space in descending order of prefix length (largest subnets first), so overflow typically happens when the sum of requested addresses exceeds the total available in the parent network. The overflowed items are shown separately in red, and the allocation table lists only the subnets that fit. Adjust your requirements (use a larger parent block or smaller subnets) and recalculate.

Does it handle /31 point-to-point links and /32 host routes?

Yes. /31 returns 2 usable addresses per RFC 3021 (effectively eliminating the network and broadcast addresses on point-to-point links). /32 returns 1 usable address for host routes. Both are supported in the parent block and as subnet requests.

How do I export my allocation plan?

Click "Copy as Text" to copy the entire allocation table to your clipboard as a tabular string you can paste into a ticket, email, or document. Click "Download CSV" to download a CSV file you can open in any spreadsheet application. Both buttons become active after the first successful calculation. The CSV includes the parent block summary and every allocated subnet.

03 / KNOWN LIMITATIONS

What Subnetsplit cannot do yet — and what it may never do.

Subnetsplit is a focused tool. These limitations are intentional or reflect infrastructure that does not yet exist:

● No IPv6 support. IPv4 only. IPv6 subnetting rules are different and not implemented.
● No saved plans across sessions. Every calculation starts from scratch when the page loads. The tool does not have a backend to persist your work. Use the export buttons before closing the tab.
● No REST API. The Team tier promises an API for automation, but the backend infrastructure to support it does not exist yet. There is no server to call.
● No team workspaces or collaboration. The Team tier description includes shared workspaces. These require a backend that is not built. Pre-orders for the Team tier are recorded but no payment is collected.
● No network reachability checks. Subnetsplit has never sent a packet, opened a socket, or looked up a route. It does arithmetic, not network scanning.
● No device configuration. The output is a table you apply yourself. Subnetsplit does not push ACLs, configure DHCP scopes, or write router configs.
● No accounts or sign-in. There are no user profiles, session storage, or authentication. This is by design — the free tier requires nothing from you.

04 / CONTACT

How to reach a person.

Email: subnetsplit-p3@agen2ic.ai

This address is monitored for support and billing inquiries related to the Subnetsplit product. Include as much detail as you can — the parent CIDR you were using, the subnet requirements, your browser and operating system, and what you expected to happen versus what actually occurred.

We aim to respond within two business days. No phone number or postal address has been established for this product.

BEFORE YOU WRITE

✓ Check that the page loads without JavaScript errors (browser dev tools > Console tab).
✓ Verify your parent CIDR is valid — Subnetsplit accepts IPv4 ranges only (e.g. 10.0.0.0/16).
✓ Search this help page for your question — the answer may be above.
✓ If the tool is not working at all, try a different browser. Subnetsplit requires a modern browser with JavaScript enabled.