FAHAQ

Wildcard Mask Calculator

Convert between subnet and wildcard masks.

Subnet mask
255.255.255.0
Wildcard mask
0.0.0.255
CIDR
/24

Free online wildcard mask calculator. Convert a subnet mask to its wildcard mask (or the reverse) and see the matching CIDR prefix — handy for Cisco ACLs and OSPF. Runs in your browser.

How to use the Wildcard Mask Calculator

  1. 01Choose whether your input is a subnet mask or a wildcard mask.
  2. 02Enter the dotted-decimal mask.
  3. 03Read off the subnet mask, wildcard mask and CIDR prefix.

Frequently asked questions

What is a wildcard mask?

A wildcard mask is the bitwise inverse of a subnet mask. Cisco access control lists and OSPF use it, where a 0 bit means 'must match' and a 1 bit means 'don't care'.

How do I turn a subnet mask into a wildcard mask?

Subtract each octet from 255 — for example 255.255.255.0 becomes 0.0.0.255. This calculator does it instantly and also shows the CIDR prefix.