I have with Redhat v7.?  It was a while ago.  Whatcha need?  You might get a better response from deja.com.  I know that Linux is not Unix but, close enough.

This will be your best bet
[url]www.smoothwall.org[/url]

Its a linux based firewall that runs on a dedicated box.  

Simple:

Tell all the hosts on the 2 networks that the unix box's IP (the one that matches the host's netmask (i.e. 2 networks, 192.0.16.0 and 192.0.17.0, UNIX has 192.0.16.1 and 192.0.17.1, for computer '192.0.16.44', use 192.0.16.1) is their default gateway (assuming that the UNIX system also has access to everything else you want it to see).

If it's just connecting the 2 networks to each other (as opposed to connecting them to the internet, other networks, etc...), then just add the UNIX system as an additional gateway.

Routing should occurr automatically. This is separate from firewalling (smoothwall, checkpoint, etc), so no extra software is needed...

If it refuses to route, you may have to change some sysctl or /proc values to make it work. Depends on the system, OS, and OS version.

Ricker,

 What's the OS on your router box and what are the network segments you want to route?

I'm no help on AIX, you might try google or

http://www.experts-exchange.com/Networking/Unix_Networking/

That's the firewall getting in the way.

Routing is still that simple. It's just a question of which machines need to gateway to where.

That firewall (which is essentially another router) needs to have a route to your network added to it's routing table. The best way to test this is to set up machines that are outside all firewalls, but on opposite sides of your router-box (A -> Rtr <- B). The easiest way to do this is to plug another box into the port that would go to the other company's network. When you can ping the 'outside' box from your network, the *routing* is done.

Then all you have to do is get the firewall configs right.

Anything further, and it's time to investigate using proxy servers....