Rules Facility Guide › Rules Reference › Specifying Terminal Addresses for the requester Variable › Specifying a Subnet Mask in Decimal Notation

Specifying a Subnet Mask in Decimal Notation

A subnet mask can be expressed in an alternate notation for IPv4 or IPv6 addresses.

• The mask is a decimal number between 0 and 32 for IPv4.
• The mask is a decimal number between 0 and 128 for IPv6.

This number follows the IP address. It is adjacent to the IP address, but separated from it by a single forward slash character.

In the example above, an IPv4 mask of 255.128.0.0 was specified. When that address is converted to hex, it becomes FF800000. In binary notation, the mask is 11111111100000000000000000000000. In Decimal notation, the mask is simply the number of one bits on the left of the binary mask, so that 10.128.0.0+255.128.0.0 is expressed as 10.128.0.0/9, which is more compact.

An IPv6 specification in a rule of FD00:7A06:A33:3::F5F3/112 would match any IP address between FD00:7A06:A33:3::0000 and FD00:7A06:A33:3::FFFF because only the first 112 bits of the address being tested would have to match the address in the rule.