IP Relay Server allows hosts on a Network to communicate with the rest of a network (such as the Internet) without revealing their own private IP Address. IP Relay Server could create four (or more) "Virtual" tunnel ports to transfer incoming data from "Real" IP address to "Virtual" IP address. General speaking, peer-to-peer file-sharing programs, chat clients such as ICQ, MSN Messenger, or Yahoo, and certain operations such as web serving and outgoing FTP require you to "Forward" or "open" ports on DSL/ADSL/Modem/Proxy router to allow them to work.

When a port is forwarded, a "Virtual Tunnel" is created which allows the request directed at port number XXXX to be "Forwarded" to a port attached to an "Internal IP address" listed on one of the network's PC.

The following diagram illustrates a basic, three-PC network connected to the Internet through an IP Relay Server.
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A small network is protected by IP Relay Server. All Internal PCs are assigned private IP addresses (e.g. 192.168.0.2, 192.168.0.3, 192.168.0.4) and access the Internet use a single shared public IP address (e.g. 71.144.14.150).
IP address of IP Relay Server is 71.144.14.150.
Private IP addresses of network PCs are: 192.168.0.2, 192.168.0.3, 192.168.0.4

After the configuration of the "port forwarding" at IP Relay Server, to connect with 192.168.0.2:1024 from Guest PC's web browser:
http://71.144.14.150:81
The next actions will take place:
Web browser will connect to a router IP address (71.144.14.150) port (81)
IP Relay Server will automatically forward all traffic to target machine IP address (192.168.0.2) port (1024).

In such way you can control machine which is located in the closed private network via Internet.