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Routing & Switching · Foundational

Wildcard Mask

An inverse mask where 0 bits must match and 1 bits are ignored, used in ACLs and OSPF/EIGRP network statements.

Definition

A wildcard mask for a subnet is usually the subnet mask subtracted from 255.255.255.255, so /24 becomes 0.0.0.255. Unlike subnet masks, wildcard bits need not be contiguous, allowing matches such as all odd subnets. 0.0.0.0 matches one host and 255.255.255.255 matches any address.

Example

network 10.1.0.0 0.0.255.255 area 0

Exam tip

Calculate wildcard as 255.255.255.255 minus the subnet mask; /27 is 0.0.0.31.

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