I'm using a SonicWall TZ 100 with a basic configuration of X0 for the LAN and X1 for the WAN. The WAN uses DHCP to obtain its routable IP address. I want to obtain a second routable IP from my ISP. I'm in luck because my cable company will provide me with an additional dynamic IP for $5/mo. How do I bind this IP to my SonicWall?

My additional dynamic IP will not be consecutive to the original one. It won't even be on the same class C. I think what I want to do is to use one of the empty ports/interfaces (X2, X3, or X4), tell that interface to use DHCP, and then add that interface to the WAN "zone". I can't figure out how to do this though. Here's what I've tried so far:

(1) I've looked in Network >> Interfaces. I see X0 and X1 but the other unused interfaces don't show up. I don't see an "Add" button to add the new interfaces.

(2) I've looked in Network >> Zones. I see that X0, X2, X3, X4 are in the LAN zone. I tried to drag X3 into the WAN zone but I can't. Nor does clicking the "Configure" button allow me to move an unused interface from LAN to WAN.

(3) I've read the post entitled Splitting up multiple WAN's on Sonicwall. This doesn't seem applicable to me.

Any thoughts?