Wildcard masks trip up a lot of people learning Cisco IOS, mainly because they're the inverse of a subnet mask and it's easy to mix the two up under exam pressure. This tool converts between the two instantly, so you can build access control lists and OSPF network statements without doing the bit-flipping in your head.
How to use the wildcard mask calculator
- Enter a subnet mask, for example 255.255.255.192, or a CIDR prefix like /26.
- The tool inverts each octet and returns the wildcard mask, in this case 0.0.0.63.
- You can also enter a wildcard mask to convert back to a standard subnet mask.
Where wildcard masks actually get used
Cisco IOS uses wildcard masks in two places you'll run into constantly: standard and extended access control lists, and OSPF network statements. In both cases, a 0 bit means "this bit must match exactly" and a 1 bit means "this bit can be anything" — which is the reverse of how a subnet mask works.
Worked example
To permit an entire 192.168.10.0/24 network in an ACL, you'd write access-list 10 permit 192.168.10.0 0.0.0.255. The wildcard mask 0.0.0.255 is the inverse of the subnet mask 255.255.255.0 — every bit in the last octet is a "don't care," meaning any host address in that range matches the line.
Mask conversions run entirely in your browser with no server requests.
