Route question

["Rod... Whitworth" <listener@witworx.com>]

I have a dialup static IP 139.a.b.c

I can dial my ISP and maintain connections for quite long times successfully.

The box doing this has an ethernet NIC which I give the IP of 192,168.1.1

Connected to this via a hub are several boxen running various oses NATted 
happily and using the net as if they had a direct connection. SFSG.

Now I want to add a box running Linux. With another 192.168.1.x address it does 
just fine BUT I want to expose it to the internet and it has its own assigned IP 
203.x.y.z

Punching a hole in my firewall rules for 203.x.y.z in and out = no problem and 
my ISP routes the 203 address via  my 139 address so outside packets get to my 
gateway.

How can I route them to the Linux box?

I tried aliasing the 203 address to the Linux NIC that already had its 192 and 
adding a route in the gateway which said to route packets for the 203 address to 
the 192.168.1.x address. I figured that they would end up on the right NIC and 
it would see them as its own but this does not work.

I know I can do this easily with a "real" router or by adding more NICs and 
giving the gateway box another 203 address but I'm sure I should be able to do 
it with one NIC each and only one 203. Can it be done? Maybe the 203 box should 
not have a 192 address as well? But then how would I tell the gateway where to 
send the packets?

None of my textbooks covers this case. :(

TIA