NAT problem with public network

[John Angelmo]

Hello

I have a network setup like this:

xl0: External:213.115.251.220
xl1: DMZ: 213.115.148.64/28
xl2: Internal: 192.168.20.0/24

Now my problem seems to be that I need to get external connection for my 
Internal network but not nating the DMZ

To simplify it all /etc/natd.conf has this line:
interface xl0

and to get nat to work I just use:
ipfw add divert natd log all from any to any via xl0

but that would nat all the traffic, how should I do just to use nat for 
my 192.168.20.0/24 network and not the 213.115.148.64/28 network?

/John