Free tool
NetworkingWildcard Mask Calculator
Convert a subnet mask or CIDR prefix to a Cisco wildcard mask instantly, with ACL and OSPF examples you can copy into a lab.
Runs in your browser — your data stays on your device
Free toolWildcard masks trip up a lot of people learning Cisco IOS, mainly because they are the inverse of a subnet mask and it is easy to mix the two up under exam pressure. This free wildcard mask calculator converts between the two instantly, so you can build access control lists and OSPF network statements without doing the bit-flipping in your head. Beginners use it for subnetting practice; network engineers use it as a fast sanity check before they paste a line into production.
How to use the Wildcard Mask Calculator
- 1
Choose subnet mask → wildcard, then enter a dotted mask such as 255.255.255.192 or a CIDR prefix like /26.
- 2
Read the matching wildcard mask (for /26 that is 0.0.0.63) plus the ready-to-copy ACL and OSPF examples.
- 3
Or switch to wildcard → subnet mask when you already have a Cisco wildcard and need the standard mask back.
Where wildcard masks actually get used
Cisco IOS uses wildcard masks in two places you will 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. That difference is why a dedicated calculator helps during CCNA revision and day-to-day network administration.
IPv4 wildcard mask example
To permit an entire 192.168.10.0/24 network in an ACL, you write access-list 10 permit 192.168.10.0 0.0.0.255. The wildcard mask 0.0.0.255 is the inverse of 255.255.255.0, every bit in the last octet is a “don’t care,” so any host in that range matches the line.
Guides and next steps
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Mask conversions run entirely in your browser with no server requests.
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