Running public IP's inside an RFC 1597 network

[Hexren]

DP> Hello,

DP> I'm running a typical Class C RFC 1597 network in my lab.  What I want
DP> to do is create another network, accessible from my private addresses,
DP> that use public IPs.  The public IPs exist in the wild but I want to have
DP> an isolated environment where I can test what happens in public space, in
DP> my lab, before I deploy changes.

DP> All the machines in question are running 5.3-STABLE.

DP> What I've setup so far are two test servers, host1 (H1) and host2 (H2)
DP> with public IPs, and a gateway (GW) machine with one public IP and one
DP> private IP.  All three machines are on a switch, the gateway has two
DP> NICs, one on the public switch and one on the private switch.

DP> e.g.,
   
DP>    External IP        Internal IP     Defaultrouter IP
DP> --------------------- --------------  ---------------
DP> GW 123.456.789.1/24   10.20.30.40/24  10.20.30.1
DP> H1 123.456.789.154/24                 123.456.789.1
DP> H2 123.456.789.161/24                 123.456.789.1


DP> I can ping between the 3 "public" IP's fine until I turn on the GW
DP> interface with the private IP.  At that point, the GW cannot ping the
DP> two "public" servers. 

DP> Obviously I'll need NAT'ing from the GW to H1 and H2 if I want packets
DP> from other hosts on my private network to see the "public" servers.
DP> What I can't figure out is how to tell my GW machine that packets
DP> destined for the 123.456.789.0/24 network are to go through my other
DP> NIC, not out through the GW's default router.

DP> I hope I've explained the situation clearly.  Googling and reading the
DP> friendly manuals has not revealed a solution to me.


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Can you provide the output of "netstat -nr", please.
Once before turning on the Internal NIC of the GW and once after that.

Thank you
Hexren