Netmask to CIDR Converter

Validate an IPv4 subnet mask and recover its prefix length.

Local calculation · inputs are not sent to a server.

Result

Calculating the example…

How it works

A valid CIDR mask is a run of one bits followed by a run of zero bits. Count the leading ones to obtain the prefix. Reject a one bit that appears after a zero.

Worked example

255.255.254.0 contains 8 + 8 + 7 leading ones, so it is /23. Two adjacent /24 address blocks fit inside an aligned /23.

Inputs, edge cases & common mistakes

255.0.255.0 is not a contiguous subnet mask. Do not paste an ACL wildcard here: 0.0.0.255 is a wildcard for /24, not its subnet mask.

Technical references

RFC 4632

Reviewed 9 September 2026. Found an issue? Send a correction with a reproducible example.