Comprehensive Technical Explanation:

In WatchGuard Fireware, a Static NAT (SNAT) action requires the public-facing external IP address to belong to the Firebox so the appliance can respond to ARP requests on the ISP subnet and accept incoming packets addressed to that destination. When defining the external IP in an SNAT dialog, the drop-down menu populates exclusively with IP addresses configured on the Firebox’s external interfaces.

If an ISP provides a block of multiple public IP addresses (such as a /29 subnet with 5 usable public IPs), the primary IP of the external interface consumes only one address. To make the remaining four public IP addresses available across Fireware configuration dialogs—including the SNAT interface list—the administrator must navigate to Network > Interfaces, edit the external interface, and add the extra public IP addresses as Secondary IP Addresses. Once configured as secondary IPs on the external interface, they immediately appear in the drop-down selection list when creating or modifying Static NAT actions.