SubnetCalc
Calculate subnet

Mask validation

Convert a subnet mask to CIDR

Enter a contiguous dotted-decimal subnet mask to find its CIDR prefix and calculate the matching IPv4 network.

Private browser calculation

Address, prefix, and host sizing

Stays on this page

Calculation mode

IPv4 address and prefix

Use a separate prefix or mask, or enter address/prefix and leave the second field empty.

A /31 result is blocked until you explicitly confirm its two endpoint semantics. Other prefixes ignore this choice.

Exact unsigned 32-bit arithmetic runs locally. This tool does not query, scan, locate, or test any address, and it does not configure a device.