Mask Input Fields
Wildcard Results
| Subnet Mask | 255.255.255.0 |
|---|---|
| Wildcard Mask | 0.0.0.255 |
| Binary Subnet Mask | 11111111.11111111.11111111.00000000 |
| Binary Wildcard Mask | 00000000.00000000.00000000.11111111 |
Subnet to Wildcard Converter
Calculating Wildcard Masks (inverse masks) can be error-prone. Our tool instantly converts any standard Subnet Mask or CIDR prefix into the exact Wildcard format needed for Cisco Access Control Lists (ACLs), OSPF area configurations, and EIGRP network statements.
Inverse Mask Logic
ACL & OSPF Ready
Binary Precision
How to Use the Wildcard Mask Converter
Converting Subnets to Wildcards
A wildcard mask is essentially the "inverse" of a subnet mask. It is primarily used in Cisco IOS for Access Control Lists (ACLs) and routing protocols like OSPF to specify which parts of an IP address should be examined or ignored.
Step 1 — Input your Mask or CIDR
You can calculate your wildcard mask using two different methods depending on the information you have:
- 1Subnet Mask: Enter the traditional decimal mask (e.g.,
255.255.255.240). - 2CIDR Slider: Drag the slider to the desired prefix length (e.g.,
/28).
The Math Trick: You can manually find a wildcard mask by subtracting each octet of the subnet mask from
255. (Example: 255 - 255 = 0, and 255 - 0 = 255).Step 2 — Review Binary Logic
The results panel will show the binary breakdown. This is crucial for understanding how the router processes the mask:
| Binary Bit | Wildcard Meaning | Subnet Meaning |
|---|---|---|
| 0 | Match: Check this bit. | Network: This bit belongs to the network. |
| 1 | Ignore: Don't check this bit. | Host: This bit belongs to the host. |
Frequently Asked Questions
What is the most common use for a Wildcard Mask?
Wildcard masks are most commonly used in Access Control Lists (ACLs) to permit or deny specific traffic ranges. They are also used in OSPF configuration to define which interfaces should participate in the routing process based on their IP address.
Is a Wildcard Mask always the inverse of a Subnet Mask?
In most standard networking scenarios (like OSPF or basic ACLs), yes. However, Wildcard masks are actually more flexible. Unlike subnet masks, which must have contiguous 1s, wildcard masks can technically have non-contiguous 0s and 1s to match complex patterns, though this is rare in modern design.
What does 0.0.0.0 mean in a Wildcard Mask?
A wildcard mask of 0.0.0.0 tells the router to "match every bit." This is used when you want to specify a single, exact Host IP. In Cisco CLI, this is often abbreviated using the keyword host.