Let me make sure I have this right, you setup is as follows:
Clients/switch ------P1 ----Xserver ---- P2 ---- ADSL--->Internet
I have a similar situation in my office using Linux. This turn the Xserver into a firewall. But to get it to work properly you need to have a different IP address...
This is a shortcoming of the web admin tool. Read up on the use of the ServerAlias directive for the apache server. You will need hand edit the site configuration files (in /etc/httpd/sites) to add this directive to the config files and restart the Apache server.
I've set up my multisite...