What is wild card mask of 32?

The wildcard mask for a /32 (slash 32) prefix is 0.0.0.0.

A wildcard mask is a numerical expression used in networking to define which parts of an IP address should be matched and which parts should be ignored. Unlike a subnet mask, where 1s define the network portion and 0s define the host portion, a wildcard mask uses 0s to signify "match this bit exactly" and 1s to signify "ignore this bit."

Understanding Wildcard Masks and /32

The /32 notation, stemming from CIDR (Classless Inter-Domain Routing), specifies a network where all 32 bits of the IP address are designated as the network portion. This is equivalent to a subnet mask of 255.255.255.255, meaning there are no available bits for host addressing. Consequently, a /32 always refers to a single, specific IP address.

To determine the wildcard mask from a given subnet mask, you subtract the subnet mask from 255.255.255.255. For a /32 prefix:

• Subnet Mask: 255.255.255.255
• Wildcard Mask Calculation: 255.255.255.255 - 255.255.255.255 = 0.0.0.0

The relationship between common CIDR prefixes, their corresponding subnet masks, and wildcard masks can be seen in the table below:

Practical Applications of a 0.0.0.0 Wildcard Mask

When a wildcard mask of 0.0.0.0 is paired with an IP address, it instructs networking devices to precisely match that particular IP address. This is because every bit in the wildcard mask is a 0, enforcing an exact match for every corresponding bit in the IP address.

Key scenarios where a 0.0.0.0 wildcard mask is used include:

• Access Control Lists (ACLs): In network security configurations, especially on routers and firewalls, a 0.0.0.0 wildcard mask is used to permit or deny traffic for a single host. For example, an ACL rule like permit ip host 192.168.1.1 is functionally identical to permit ip 192.168.1.1 0.0.0.0, ensuring only traffic from or to that specific IP address is considered.
• Routing Protocol Configuration: In some routing protocols, such as OSPF (Open Shortest Path First), a 0.0.0.0 wildcard mask can be used to explicitly enable OSPF on a specific interface IP address, rather than on an entire subnet.
• Precise Packet Filtering: Firewalls and other packet filtering systems leverage the 0.0.0.0 wildcard mask for highly granular control, allowing administrators to filter traffic destined for or originating from just one IP address.

Ultimately, a /32 prefix combined with a 0.0.0.0 wildcard mask provides the highest level of specificity, uniquely identifying a single IP address without any ambiguity.