Setting up a jail's IP address?

[Luke]

I'm attempting to set up my first jail.  This box acts as a firewall for 
my network.  It contains two network cards - one points to my internal 
network and the other to the outside world.
I want to run SSH in a jail to allow SSH access from the outside world to 
a controlled environment inside this box.

The first problem I'm having is that according to every jail tutorial I've 
seen so far, I'm supposed to bind the IP address of the jail to a NIC. 
Well... both of my NICs already have IP addresses bound to them, so 
ifconfig always fails with "ioctl (SIOCAIFADDR): File exists".

The second problem is that I'm not entirely sure which network card I 
would bind the jail to anyway.  The jail has to have an internal IP 
address, I assume, but it needs to talk to the outside world so my first 
instinct is to bind it to the network card that's hooked to the outside 
network.  That would mean binding two addresses that aren't even on the 
same network to one card, and that sounds like it might be problematic.

Can someone clear this up for me?