Use a 31-bit or 32-bit Subnet Mask
To conserve IPv4 addresses, some Internet Service Providers assign IPv4 addresses with a /31 or /32 subnet mask. If your Firebox is configured in mixed routing mode, you can configure an external interface to use an IPv4 address with a 31-bit or 32-bit subnet mask.
When you configure an IP address, you specify the subnet mask in slash notation. For information about slash notation, go to About Slash Notation.
31-bit Subnet Mask (/31)
In mixed routing mode, you can configure any external interface to use an IPv4 address with a 31-bit subnet mask. A 31-bit subnet mask is often used for an interface that is the endpoint of a point-to-point network. The use of 31-bit subnet masks for IPv4 point-to-point links is described in RFC 3021.
32-bit Subnet Mask (/32)
A 32-bit subnet mask defines a network with only one IP address. In mixed routing mode, you can only configure a 32-bit subnet mask for a physical external interface. Because you cannot configure a virtual external interface with a default gateway on a different subnet, you cannot use a /32 subnet mask for a virtual external interface, such as a VLAN or Link Aggregation interface.