TCP Problem

Can anyone point me in the right direction here.

We are having a problem PINGing our AS/400 via a NAT translation in the
firewall.  The long term intent is to allow certain HTTP transactions to
come through the wall and be served up by the AS/400.  As part of testing
this we have one of the PCs set up to appear to the firewall to be coming
from outside.  He can ping any of the PCs (and a Linux box) but the AS/400
just doesn't respond.  If the 400 is pinged from inside the firewall or
from outside via the VPN it responds just fine.

Any thoughts?  Any kind of logging I can turn on that would show me what is
going on?  I'm somewhat of a TCP/IP virgin so any help welcome.

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