IP Subnet Calculator

Why do /31 and /127 show both addresses as usable?

RFC 3021 defines IPv4 /31 as a point-to-point link prefix with no broadcast address, so both addresses in the block are host-usable instead of one being reserved as network and the other as broadcast. IPv6 /127 follows the same convention. Every other prefix length keeps the ordinary network-address/broadcast-address-reserved behavior.

Why is the reverse-DNS zone sometimes shown with a note?

A clean reverse-DNS zone name only exists at whole-octet boundaries for IPv4 (/8, /16, /24, /32) or whole-nibble boundaries for IPv6 (every multiple of /4). When your prefix falls in between, the zone shown is truncated to the nearest fully covered boundary and flagged with an inline note — building the full classless-delegation name (RFC 2317) for in-between prefixes is out of scope for this tool.

What do the /48, /56, and /64 subdivision buttons do?

For IPv6 blocks between /48 and /64, the tool suggests common next-step subnet sizes. Clicking one replaces the CIDR input — and the URL — with the first available sub-block at that prefix length and recomputes the whole result for it, the same one-CIDR-in, one-result-out behavior as typing a new value directly. There's no separate drill-down view or back-to-parent history to manage.

Can I share a specific subnet result with a teammate?

Yes — every CIDR you enter, or subdivision you click, updates the page URL (for example /tools/subnet?cidr=192.168.1.0%2F24). Copy that URL and send it to anyone; opening it reproduces the exact same result immediately, with no login or setup required.

Does this tool send the CIDR I enter anywhere?

No. All subnet math runs entirely in your browser using native BigInt arithmetic — nothing you type is sent to a server or logged, and the CIDR value is deliberately kept out of analytics.