The CIDR to Wildcard Converter is an essential tool for network engineers working with Cisco IOS access control lists. While subnet masks use 1s to represent network bits, Cisco ACL wildcard masks use 1s to represent host bits, the inverse. This means a /24 subnet mask of 255.255.255.0 has a wildcard mask of 0.0.0.255. Converting between these manually is error-prone, especially for unusual prefix lengths.

This tool works in both directions, enter a CIDR prefix to get the wildcard, or enter a wildcard mask to find the CIDR. It also shows the full subnet details including network address, broadcast address, first and last usable hosts, and the binary representation of both masks side by side so you can see exactly which bits are network bits and which are host bits.

The Cisco ACL Syntax Generator produces ready-to-paste IOS configuration lines for standard ACLs, extended ACLs and OSPF network statements, saving you from having to remember the exact syntax under pressure.