Also known as: inverse mask
Quick Definition
The inverse of a subnet mask, used in ACLs and OSPF network statements.
A wildcard mask is the bitwise inverse of a subnet mask. Where the subnet mask has 1s in the network portion, the wildcard mask has 0s (meaning 'must match'). Where the subnet mask has 0s in the host portion, the wildcard mask has 1s (meaning 'don't care'). Wildcard masks appear in ACLs and in OSPF network statements. A wildcard mask of 0.0.0.255 matches any host in a /24 subnet.
To permit all hosts in 192.168.1.0/24: access-list 1 permit 192.168.1.0 0.0.0.255. The wildcard 0.0.0.255 means the first three octets must match exactly, but the last octet can be anything.
Wildcard masks are NOT subnet masks. 0.0.0.255 is the wildcard for a /24, but it is NOT the subnet mask (which would be 255.255.255.0). Confusing the two is one of the most common CCNA mistakes.
A wildcard mask is the bitwise inverse of a subnet mask. Where the subnet mask has 1s in the network portion, the wildcard mask has 0s (meaning 'must match'). Where the subnet mask has 0s in the host portion, the wildcard mask has 1s (meaning 'don't care'). Wildcard masks appear in ACLs and in OSPF network statements. A wildcard mask of 0.0.0.255 matches any host in a /24 subnet.
Wildcard masks are NOT subnet masks. 0.0.0.255 is the wildcard for a /24, but it is NOT the subnet mask (which would be 255.255.255.0). Confusing the two is one of the most common CCNA mistakes.
To permit all hosts in 192.168.1.0/24: access-list 1 permit 192.168.1.0 0.0.0.255. The wildcard 0.0.0.255 means the first three octets must match exactly, but the last octet can be anything.
Wildcard Mask falls under the IP Addressing domain of the 200-301 exam. Understanding it in context with related terms like acl and subnet-mask is essential for answering scenario-based questions correctly.