SMC Networks AR4505GW - Manual (Page 69)

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Virtual Server
If you configure the ADSL Router as a virtual server, remote users
accessing services such as web or FTP at your local site via public IP
addresses can be automatically redirected to local servers configured with
private IP addresses. In other words, depending on the requested service
(TCP/UDP port number), the ADSL Router redirects the external service
request to the appropriate server (located at another internal IP address).
For example, if you set Type/Public Port to TCP/80 (HTTP or web) and
the Private IP/Port to 192.168.2.2/80, then all HTTP requests from
outside users will be transferred to 192.168.2.2 on port 80. Therefore, by
just entering the IP address provided by the ISP, Internet users can access
the service they need at the local address to which you redirect them.
A list of ports is maintained at the following link:
http://www.iana.org/assignments/port-numbers.