The term Mask IP address means that my router will give intruders the configured IP address(not the real IP) and original IP address means the real IP address.

If you are running WindowsXP with Service Pack 2 and you have installed the Norton Firewall, there would be no alert on your Norton Firewall because the Windows Firewall is doing the job for you. In my case, 2 firewalls(Router/Windows)+1 safety net firewall(Norton) is doing the job.