The external interface is the interface that connects to the Internet or a Wide Area Network (WAN). The external interface must have an IP address to operate correctly.
You can assign a static or dynamic IP address to the external interface. Dynamic IP addresses do not stay the same, and are assigned with Dynamic Host Configuration Protocol (DHCP) or PPPoE (Point-to-Point Protocol over Ethernet). Static IP addresses always stay the same, and are usually assigned manually.
A static IP address is necessary for the Firebox to use some functions. If you do not have all of the information you need to configure your external interface, speak to your network administrator or Internet Service Provider (ISP).