Firewall

Hi,

I will try to describe the environment what I want to test.
I set up a firewall to split two networks (A and B). Both with
different IP-Address ranges.
From network-A I want to be connected, trough the firewall in
network-B. In network-B I have to authenticate at a proxy. After
authentication all access to the servers in network-B is running
through the proxy.

The proxy uses the servername a.b.c.domain-A.de to forward my access
to a server. But my client in network A doesn't know this address of
the server.
Replacing the servername a.b.c.domainA.de with the firewall-ip in the
URL the access to the server is ok.
How can I reach these differnt servers through the firewall and trough
the proxy without changing the URL in my client?

I hope that this is not a too stupid question with a too easy
description of the complex environment,...
thanks for help Juergen