Wildcard Mask Calculator
Calculate the wildcard mask (inverse subnet mask) for any IP address and CIDR prefix — used in Cisco ACLs and OSPF configuration.
- Free to use
- Accurate results
- No registration required
- Works on all devices
Wildcard mask = 255.255.255.255 minus the subnet mask. Enter an IP and CIDR prefix to calculate it.
Your result
0.0.0.255
Wildcard mask
Subnet mask255.255.255.0
Network address192.168.1.0
AI explanation
Formula
Wildcard mask = 255.255.255.255 − subnet mask (octet by octet)Worked example
192.168.1.0 / 24
| Field | Value |
|---|---|
| IP address | 192.168.1.0 |
| CIDR prefix (/n) | 24 |
| Wildcard mask | 0.0.0.255 |
| Subnet mask | 255.255.255.0 |
| Network address | 192.168.1.0 |
Assumptions
- IPv4 only.
- Covers only the simple "inverse of a contiguous subnet mask" case — Cisco ACLs also support discontiguous wildcard masks for advanced matching, which this calculator doesn't generate.
Frequently asked questions
What is a wildcard mask used for?
Cisco IOS access control lists (ACLs) and OSPF network statements use wildcard masks instead of subnet masks — a 0 bit means "must match," a 1 bit means "don't care."
How is a wildcard mask different from a subnet mask?
A wildcard mask is the bitwise inverse of the corresponding subnet mask — where the subnet mask has a 1, the wildcard mask has a 0, and vice versa.
Why does 255.255.255.0 become 0.0.0.255?
Each octet is subtracted from 255 — 255−255=0 for the first three octets, and 255−0=255 for the last, since the /24 mask has all-1s in the first three octets and all-0s in the last.