D-Link DI-707P - Manual (Page 21)

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Virtual Server
The firewall filters out unrecognized packets to protect your Intranet; so all
computers networked with the DI-707P are invisible to the outside world. If you
wish, you can make some of them accessible by enabling the Virtual Server
Mapping.
A virtual server is defined as a Service Port, and all requests to this port will be
redirected to the computer specified by the Server IP.
For example, if you have an FTP server (port 21) at 192.168.0.1, a Web server
(port 80) at 192.168.0.2, and a VPN server at 192.168.0.6, then you need to
specify the following virtual server-mapping table:
Service Port Server IP Enable
21 192.168.0.1 V
80 192.168.0.2 V
1723 192.168.0.6 V