Wildcard masks look like subnet masks but work in reverse, and they show up in exactly the configuration contexts where getting that reversal wrong causes an access control list or OSPF network statement to match the wrong addresses. This wildcard mask calculator takes an IP address and a wildcard mask and returns the first and last address in the resulting range, so you can confirm exactly what a wildcard mask actually covers before deploying it.

How to use this wildcard mask calculator

1. Enter an IP Address in the input field, for example 10.0.0.1.
2. Select a Wildcard Mask from the dropdown, for example 0.0.0.255.
3. Review the First Address and Last Address fields, showing the full range of addresses that IP and wildcard mask combination covers.

What this wildcard mask calculator does

A wildcard mask defines which bits of an address must match exactly and which bits are allowed to vary, or are “wild.” This calculator takes your entered IP address and wildcard mask, applies the wildcard logic to that address, and returns the lowest and highest address in the resulting matched range, giving you a concrete first-to-last address span instead of leaving you to work out the range from the raw mask bits yourself.

How wildcard masks work

This is the exact inverse of how a standard subnet mask works, where a 1-bit means “part of the network identifier” and a 0-bit means “part of the host portion.” Because of this inversion, a wildcard mask of 0.0.0.255 matches any address in a /24-sized block, since the final octet is entirely wild while the first three octets must match exactly, the equivalent range a 255.255.255.0 subnet mask would define, just expressed in reversed logic.

Wildcard mask versus subnet mask

Because the two masks are bitwise inverses of each other for the same address block, converting between them is straightforward (255.255.255.255 minus the subnet mask equals the wildcard mask, and vice versa), but the two are not interchangeable in configuration syntax. Using a subnet mask where a device expects a wildcard mask, or the reverse, will produce a match that’s the exact opposite of what was intended, which is a common source of ACL misconfiguration.

Why wildcard masks matter for ACLs and OSPF

Access control lists on many router and firewall platforms, along with OSPF network statements, specify address matching using wildcard masks rather than subnet masks. This means a network engineer writing an ACL rule or an OSPF network statement needs to think in wildcard terms directly, and a small mistake, like using 255.255.255.0 instead of 0.0.0.255, results in a rule that matches only a single specific host rather than an entire subnet, or vice versa. Checking the resulting First Address and Last Address range with this calculator before deploying a configuration is a quick way to confirm the wildcard mask actually covers the addresses it’s meant to.

Non-contiguous wildcard masks

Unlike subnet masks, which always consist of a contiguous block of 1-bits followed by 0-bits, wildcard masks are sometimes written with non-contiguous bit patterns for advanced matching scenarios, such as matching every other subnet or a specific bit pattern across multiple octets. While the most common wildcard masks mirror standard subnet mask boundaries (0.0.0.255, 0.0.0.15, and so on), the wildcard format itself supports more flexible patterns than a standard subnet mask can express, which is part of why it’s used in ACL contexts that sometimes need that extra flexibility.

Frequently asked questions