I installed ISPconfig on my network, behind a router. And my router has forwarding port direct to the server I installed ISP. I can open ISP through http://192.168.0.100:81 but can not open it on http://www.xyz.com:81. The server is connected to out side, it show apache test page on http://www.xyz.com. Port 81 is open to out side. What could be the problem?