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Wildcard Mask Calculator

Convert any CIDR prefix into the wildcard (inverse) mask used by Cisco ACLs and OSPF network statements, with the binary shown and a ready-to-paste ACL line.

Runs locallyComputed in your browser: nothing is sent anywhere

tools.wildcard runs locally

When you’d reach for this

Two places demand wildcards: Cisco ACL entries and OSPF network statements. Getting one octet wrong silently matches the wrong networks, so check here first, then read why the inversion exists at all.

Learn the idea behind the tool

This tool is the fast answer; the lesson is the understanding. Start with ACLs in IOS.

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