Wildcard Mask Calculator (ACL)
Bidirectional conversion between subnet masks and Cisco/OSPF wildcard inverse masks with ACL rule snippets.
Wildcard Masks vs Subnet Masks: Key Differences
In enterprise routing and firewall operations, misunderstanding wildcard masks is one of the most common causes of misconfigured access lists and accidental traffic blackholing.
Subnet Mask Logic
Used by IP stack routing tables (e.g. 255.255.255.0). Binary 1 = Network portion. Binary 0 = Host portion. Identifies which segment of an address represents the broadcast domain.
Wildcard Mask Logic
Used by Cisco ACLs and OSPF routing statements (e.g. 0.0.0.255). Binary 0 = Match bit exactly. Binary 1 = Ignore bit (wildcard). Tells packet filters which bits to inspect.
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Frequently Asked Questions
What is a wildcard mask and why is it also called an inverse mask?
A wildcard mask is an inverted bitmask used by networking protocols like Cisco IOS Access Control Lists (ACLs) and OSPF to match one or more IP addresses. In a regular subnet mask, binary 1s indicate network bits that must match, while binary 0s represent host bits. In a wildcard mask, the logic is reversed: binary 0s mean "must match exact bit value", and binary 1s mean "ignore this bit (wildcard)".
How do you calculate a wildcard mask from a subnet mask?
The easiest calculation is to subtract each octet of the standard subnet mask from 255. For example, for a /26 subnet mask of 255.255.255.192: 255 - 255 = 0, 255 - 255 = 0, 255 - 255 = 0, and 255 - 192 = 63. Thus, the wildcard mask is 0.0.0.63.
Can a wildcard mask have non-contiguous bits?
Yes. While standard subnet masks must have contiguous 1s followed by contiguous 0s, Cisco Access Control Lists allow non-contiguous wildcard masks. For instance, a wildcard mask of 0.0.0.1 can match all even or all odd IP addresses within a subnet, although modern network best practices recommend adhering to standard contiguous boundaries for security auditing.
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