One focused calculator

About Cidrspan

Cidrspan turns one entered IPv4 address and prefix into an inspectable mathematical block. It is useful when a network note contains an address with nonzero remainder bits and you want to see the canonical prefix, mask, inclusive endpoints and count. The entered address stays visible separately from its block's lower address.

The tool uses exact unsigned arithmetic over the complete 32-bit address range. It handles /0 through /32 without signed bitwise overflow and without creating an address list. A bounded bit diagram explains which leading bits remain fixed and which remainder bits are zeroed in the canonical lower address.

A deliberately limited result

This is not a network test. It does not scan an address, resolve a hostname, read your network settings or decide a routing policy. It does not look up ownership, allocation, reachability or private/global status. Its full address count is not a universal deployable-host capacity, and its highest numerical address is not unconditionally labeled a broadcast destination.

That distinction matters especially for /31 and /32. Different operational contexts have different endpoint rules. The guide explains those limits and links to the primary RFCs supporting the width, prefix notation and point-to-point example. It avoids converting a mathematical boundary into a configuration recommendation.

Free use and feedback

One independent website contains this one calculator and its supporting guide, examples, privacy and contact pages. No sign-in, payment or email is required to inspect a block. This source release has no active analytics, advertising or consent-platform loader. Source preparation and website publication do not establish advertising approval, visitors or earnings.

Optional feedback uses an existing shared public forwarding alias. If you find an issue, an invented documentation-range example, the prefix you entered and the expected numerical output are enough. There is no need to send real infrastructure details. The contact page explains how to report a reproducible issue, while the privacy page describes local input processing and separate hosting requests.