If you have multiple IP Addresses for your WAN connection, you can use all of them by creating Aliases.
As an example, we will assume that you have a block of five static IP addresses from your ISP:
xxx.xxx.xxx.100 xxx.xxx.xxx.101 xxx.xxx.xxx.102 xxx.xxx.xxx.103 xxx.xxx.xxx.104
First, configure your Red interface with the first address:
xxx.xxx.xxx.100 using the setup program on the console. Don't forget to specify your gateway and DNS servers! Your Red interface should now work properly, but outside users cannot connect to you using the other four addresses.
To enable the remaining addresses, follow these steps:
You can now use these aliases in Port Forwarding rules and elsewhere in the system to control the flow of traffic.