31-bit subnet masks

A 31-bit subnet mask includes only two addresses for point-to-point connections.

31-bit subnet mask implementation

For routed interfaces, you can configure an IP address on a 31-bit subnet for point-to-point connections. Normally, the first and last address in a 2-address subnet is reserved for the network and broadcast, so a 2-address subnet is not usable. However, if you have a point-to-point connection and do not need network or broadcast addresses, a 31-bit subnet is a useful way to preserve addresses in IPv4.

31-bit subnet mask use cases

The failover link between two Firewall Threat Defenses only requires two addresses. Any packet that is transmitted by one end of the link is always received by the other, and broadcasting is unnecessary. You can also have a directly-connected management station running SNMP or Syslog.