Wildcard Mask Calculator

Prefix /24: netmask 255.255.255.0, wildcard 0.0.0.255.

How it works

• Resolved the input to a prefix length.
• Built the netmask from the prefix as an unsigned 32-bit integer.
• Inverted every bit to produce the wildcard mask.

wildcard = NOT netmask

• A wildcard mask is the bitwise complement of the subnet mask.
• Only contiguous netmasks have a defined prefix length and are accepted.

Methodology

1. Parse the input as a netmask/prefix or as a wildcard mask.
2. Invert a wildcard to recover the netmask and confirm the netmask is contiguous.
3. Derive the prefix length, netmask, wildcard and host counts from the resolved boundary.

Usage

1. Choose whether the input is a netmask/prefix or a wildcard mask.
2. Enter the value.
3. Read the prefix, netmask and wildcard.

Examples

These are examples, not current values: they use fixed inputs, while the interactive tool computes live results in your browser.

A /24 netmask to wildcard

{
  "kind": "netmask",
  "mask": "255.255.255.0"
}

A wildcard back to a prefix

{
  "kind": "wildcard",
  "mask": "0.0.0.255"
}

A /30 from prefix notation

{
  "kind": "netmask",
  "mask": "/30"
}

Limitations

Why routers use inverse masks

Access control lists express a match as a base address plus a wildcard mask: a 1 in the wildcard means the corresponding address bit is ignored, and a 0 means it must match. That is exactly the complement of the subnet mask.

Because wildcards are just bit patterns, they can be non-contiguous, but a non-contiguous wildcard no longer corresponds to a single subnet and has no prefix length. This tool states that boundary instead of inventing a prefix.