Why do routers use wildcard masks instead of subnet masks?

Wildcard masks let Cisco ACLs specify which bits to ignore when matching addresses, offering more flexible (non-contiguous) matching than a plain subnet mask.

Wildcard Mask Calculator

Wildcard masks are the inverse of subnet masks and are commonly used in Cisco access control lists (ACLs) and OSPF configuration.

Inputs

Results

Subnet Mask: 255.255.255.0
Wildcard Mask: 0.0.0.255

Try an example

Formula

Wildcard Mask = 255.255.255.255 − Subnet Mask (bitwise inverse)

Learn How This Works

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