CIDR to Netmask Converter

Convert an IPv4 prefix length into a contiguous dotted-decimal subnet mask.

Local calculation · inputs are not sent to a server.

Result

Calculating the example…

How it works

Set the first p of 32 bits to one and the rest to zero. Convert each eight-bit group into a decimal octet. This mask separates the network bits from host bits.

Worked example

/27 has 24 full one bits followed by 11100000, giving 255.255.255.224. Its block size is 32 addresses.

Inputs, edge cases & common mistakes

/0 produces 0.0.0.0 and /32 produces 255.255.255.255. A prefix is a bit count, not the number of hosts. IPv6 uses prefix lengths directly rather than IPv4-style dotted masks.

Technical references

RFC 4632

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