Sometimes your ISP will provide you with additional external public IPs that can be used for the routing of additional services internally.
To add additional IPs on a WAN port edit the WAN ports settings (Network -> Connection Settings) and fill in the additional IP’s in the ‘Additional Public IP Settings’ section. Click Save and Apply.
Now when you add a new service for Inbound Access (Network -> Inbound Access -> Services) in the ‘Inbound IP Address(es)’ Section you can select which of the available WAN IPs the service should be port forwarded through.