About Slash Notation

Your Firebox or XTM device uses slash notation for many purposes, including policy configuration. Slash notation, also known as CIDR (Classless Inter-Domain Routing) notation, is a compact way to show or write a subnet mask. When you use slash notation, you write the IP address, a forward slash (/), and the subnet mask number.

To find the subnet mask number:

1. Convert the decimal representation of the subnet mask to a binary representation.
2. Count each “1” in the subnet mask. The total is the subnet mask number.

For example, you want to write the IP address 192.168.42.23 with a subnet mask of 255.255.255.0 in slash notation.

1. Convert the subnet mask to binary.
   In this example, the binary representation of 255.255.255.0 is:
   11111111.11111111.11111111.00000000.
2. Count each "1" in the subnet mask.
   In this example, there are twenty-four (24).
3. Write the original IP address, a forward slash (/), and then the number from Step 2.
   The result is 192.168.42.23/24.

This table shows common network masks and their equivalents in slash notation.

See Also

About IP Addresses

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