Ian said:
How do you add the internal interface into NAT
Same way you add the external interface.
Go to NAT, right click and add interface, change the public-external to
private-internal (radio buttons) or vice versa.
NAT is smart enough to 'guess' that a NIC is likely private-internal and
a modem is likely (almost always) public-external but if it guesses wrong
you can always choose the other radio button.