About PrefixBench

PrefixBench is a free subnet calculator built for network engineers, cloud architects, and IT students. It runs entirely in your browser. No data leaves your machine.

Who builds this

PrefixBench is a one-person project, not a company. I work in electronics and hardware design for a living, and networking was never really my area. Setting up a NAS and some home automation gear is what pushed me into it, and I spent a while jumping between different subnet calculators, each one good at something and annoying at something else. Eventually I just built the version I wanted, with a lot of help from Claude Code, and put it online in case anyone else was stuck in the same place.

There is no company behind it and nothing to sell, so the goals are fairly boring: get the math right, keep it fast, keep it free. There are a couple of small ads on the page, and that is about the extent of the business model.

Features

IPv4 and IPv6: full subnet calculation for both protocols with one-click toggle
Complete per-subnet output: subnet mask, wildcard mask (for ACLs), public/private/reserved type, reverse DNS zone (in-addr.arpa / ip6.arpa), and integer/hex address forms
Split/Join Designer: interactively divide and merge subnets with a visual tree
VLSM Planner: specify host requirements and get optimally-sized, correctly-aligned IPv4 subnets
Cloud Modes: accurate IPv4 host counts for AWS (5 reserved), Azure (5), GCP (4), and OCI (3)
Export: CSV, Excel (.xlsx), JSON, clipboard copy, and shareable URLs, including every field above
Visual Block Map: proportional view of your address space allocation
Dark Mode: easy on the eyes during late-night subnet planning
Undo/Redo: Ctrl+Z / Ctrl+Y with 50-state history
Binary/Hex View: toggle address display format; binary view colors the network bits separately from the host bits

Why We Built This

Existing subnet calculators are either outdated (no cloud modes), limited (no VLSM), or have poor UX (no dark mode, no undo). We wanted a tool that covers the full workflow, from planning to export, with correct math and a clean interface.

Technical Details

Built with Astro, React, and Tailwind CSS
100% client-side: no backend, no database, no tracking of your calculations
Hosted on Cloudflare Pages (global CDN, free HTTPS)
URL sharing uses LZ-string compression. Your subnet designs are encoded in the URL, not stored on a server.

Accuracy and sources

Every calculation follows the published standards rather than rules of thumb. Address classification, reserved ranges, and reverse DNS all map to the relevant RFCs, including RFC 1918 (private address space), RFC 4632 (CIDR), RFC 3021 (/31 point-to-point links), RFC 2317 (classless reverse DNS), and RFC 5952 (IPv6 text representation). Cloud host counts use each provider's own documented reserved-IP rules: AWS and Azure reserve 5 addresses per subnet, GCP reserves 4, and OCI reserves 3.

The math is covered by an automated test suite of several hundred cases so that edge cases (a /31, a /32, the boundaries of each cloud mode) stay correct as the tool changes. If you ever find a result that looks wrong, please tell me: a calculator is only worth using if you can trust the numbers.

Educational Content

We publish free networking guides:

Contact

Questions, feedback, or corrections? Email hello@prefixbench.com or see the contact page.