Routing ideas needed

[whyzzi_(_at_)_themail_(_dot_)_com]

I've got two separate ip based networks, no automated routing system available (both are NT networks without RIP/RRAS and exist on separate subnets). Each network has a separate default gateway. In the middle, I have 

a 486PC with two 3com 509 ISA NICs. A simplified diagram looks like this:

 MyNetwork(GW:192.168.128.1)                their network(GW:192.168.100.254)
 -------------         ----------           -------------
|192.168.128.0|_______|OpenBSD2.7|_________|192.168.100.0|
|255.255.255.0|       |  i486    |ep1      |255.255.255.0|
 -------------        |          |          -------------
        192.168.128.29|ep0    ep1|192.168.100.21
                       ----------


I need to maximize security but still allow for certian ports on each side of the network. Up to this point I had successfully used Ipnat, but they've suddenly source iped (192.168.100.xxx) their links and images on their intranet sites, and although the initial connection can be handled by NAT, all subsequent clicks/pics require a route to 192.168.100.xx, which travels to the default gateway and subsequently dropped.

A bridge doesn't route the information (but I CAN get the bridging feature to work), and I can't get routed to pass the info between the interfaces (I did enable routed in rc.conf, disabled ipnat rule loading, sysctl.conf has ip.forward=1... something else I am missing? I tried a /etc/gateways file but don't think I fully understood it)

I have NOT yet tried these features together (bridge + routed SHould I?) OR should I do something else?

I have searched the archives and read man pages only to come up empty. I was hoping some else might be able to point me in the right direction.

Thanks in advance!
Peter V 
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