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Wildcard Mask

Demystifying Network Wildcard Masks

In the realm of networking, IP addresses and subnet masks play a crucial role in determining how devices communicate within a network. While subnet masks are widely understood, their lesser-known cousin, the network wildcard mask, is equally important but often shrouded in mystery.

Let’s start with subnet masks. These masks are used to divide an IP address into network and host portions. They consist of a series of ones followed by zeros in binary notation. For instance, a subnet mask of in its binary form looks like 11111111.11111111.11111111.00000000, effectively indicating that the first 24 bits are dedicated to the network, leaving 8 bits for hosts in a typical IPv4 address.

Enter the wildcard mask—a bitwise inverse of the subnet mask. This mask is used in some networking configurations, such as Access Control Lists (ACLs) in Cisco devices, to define ranges of IP addresses to permit or deny access.

Let’s break down how a wildcard mask works with an example:

Suppose we have an IP address with a subnet mask of In an ACL, if we wish to permit or deny a range of IP addresses, say from to, we can use a wildcard mask to specify this range.

The wildcard mask for this scenario would be the inverted subnet mask: (since the subnet mask is

Here’s how it operates:

  • IP Address (binary: 11000000.10101000.00000001.00001010)
  • IP Address (binary: 11000000.10101000.00000001.00010100)

The wildcard mask in binary is 00000000.00000000.00000000.11111111.

By applying this mask to the range of IP addresses, it identifies which bits can vary or be "wild." In this case, the fourth octet (the last 8 bits) of the IP addresses can vary within the range 10 to 20, while the first three octets remain fixed at 192.168.1.

Understanding wildcard masks can be immensely beneficial when configuring complex network rules, especially in scenarios where specific ranges of IP addresses need special handling.

To summarize, while subnet masks divide IP addresses into network and host portions, wildcard masks complement this by specifying ranges of addresses within a network for certain network operations or policies.

In the intricate landscape of networking, mastering wildcard masks adds another layer of precision and control, empowering administrators to fine-tune access rules with accuracy and efficiency.