Wildcard Mask Calculator

Simple conversion between subnet masks and wildcard masks

What is a Wildcard Mask?

A wildcard mask is the inverse of a subnet mask. While subnet masks use 1s to indicate network bits and 0s for host bits, wildcard masks use 0s for network bits and 1s for host bits. They're commonly used in Cisco ACLs and OSPF configurations. For example, subnet mask 255.255.255.0 becomes wildcard mask 0.0.0.255.

Enter subnet mask or CIDR notation
Enter wildcard mask

Visual Representation

Subnet Mask
255
255
255
0
Wildcard Mask
0
0
0
255
CIDR Notation
/24
Total Hosts
254

Understanding Wildcard Masks

What is a wildcard mask?

Inverse of subnet mask: A wildcard mask is the bitwise inverse of a subnet mask. Where the subnet mask has 1s, the wildcard mask has 0s, and vice versa.

Example: Subnet mask 255.255.255.0 becomes wildcard mask 0.0.0.255

Usage: Commonly used in Cisco ACLs, OSPF area configurations, and route filtering.

How to calculate wildcard masks?

Simple formula: 255.255.255.255 - Subnet Mask = Wildcard Mask

Example calculations:

  • 255.255.255.0 → 0.0.0.255
  • 255.255.254.0 → 0.0.1.255
  • 255.255.240.0 → 0.0.15.255

Common wildcard masks

Frequently used patterns:

  • /24: 0.0.0.255 (256 addresses)
  • /16: 0.0.255.255 (65,536 addresses)
  • /8: 0.255.255.255 (16.7M addresses)
  • Host: 0.0.0.0 (single address)

Cisco ACL examples

Access list syntax:

  • access-list 10 permit 192.168.1.0 0.0.0.255
  • access-list 10 deny 10.0.0.0 0.255.255.255
  • access-list 10 permit any = 0.0.0.0 255.255.255.255