Subnet Mask to CIDR

Convert dotted mask to prefix length. Rejects non-contiguous masks.

Subnet mask

CIDR prefix/24

How to read the result

Counts leading one-bits. 255.0.255.0 invalid: ones must be contiguous.

Example

255.255.255.0 → /24

FAQ

Why is 255.0.255.0 invalid?

Masks must be contiguous ones then zeros. That pattern interleaves, no valid prefix.