About Slash Notation
Your Firebox uses slash notation, also known as CIDR (Classless Inter-Domain Routing) notation, for many purposes, such as policy configuration. You use slash notation differently for IPv4 and IPv6 addresses.
Slash notation is a compact way to show or write an IPv4 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:
- Convert the decimal representation of the subnet mask to a binary representation.
- Count each “1” in the subnet mask. The total is the subnet mask number.
For example, to write the IPv4 address 192.168.42.23 with a subnet mask of 255.255.255.0 in slash notation:
- Convert the subnet mask to binary.
In this example, the binary representation of 255.255.255.0 is:
- Count each 1 in the subnet mask.
In this example, there are twenty-four (24).
- 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.
|Network Mask||Slash Equivalent|
In IPv6, slash notation is used to represent the network identifier prefix for an IPv6 network. The prefix is expressed as a slash (/) followed by the prefix size, which is a decimal number between 1 and 128. The CIDR notation works exactly the same as with IPv4, which means if you have a /48, that means the first 48 bits of the address are the prefix.
This table shows common IPv6 network prefixes and the number of IPv6 subnets and IPv6 addresses they support.
|Prefix||Number of Subnets|
|/64||1 IPv6 subnet with up to 18,446,744,073,709,551,616 IPv6 host addresses|
|/56||256 /64 subnets|
|/48||65,536 /64 subnets|
A network site that is assigned a /48 prefix can use prefixes in the range /49 to /64 to define valid subnets.