How does a prefix length map to a subnet mask?

The prefix is the number of leading 1 bits. /24 sets the first 24 bits to 1, giving 11111111.11111111.11111111.00000000 = 255.255.255.0.

What is a wildcard mask?

It is the bitwise inverse of the subnet mask (255.255.255.0 becomes 0.0.0.255). Cisco access lists and OSPF use wildcard masks to match address ranges.

Why does my mask show an error?

A valid IPv4 subnet mask must have all its 1 bits contiguous from the left. Values like 255.0.255.0 are rejected because the 1 bits are not contiguous.

Subnet Mask Converter

Convert between CIDR prefix and dotted-decimal subnet mask.

Convert between CIDR prefix and subnet mask

Switch in either direction: a CIDR prefix (/24) to a dotted-decimal mask (255.255.255.0), or a mask back to its prefix. The tool also shows the wildcard mask and the number of usable hosts the mask allows.

Mask-to-prefix conversion validates that the mask is contiguous, so invalid masks are flagged rather than silently accepted. Everything runs in your browser.