This option is only available if you set the red-interface to static.
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:
First, use the setup program on the console to configure your Red interface with a static IP address of 220.127.116.11 (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.