TRENDnet TDM-E400 - Manual (Page 129)

Chapter 12. Configuring Network Address Translation
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A port ID identifies the specific function of the computer
connected to it, and therefore can limit the types of data that
pass to and from the computer. For example, Web (HTTP)
servers are usually identified by port number 80; packets
containing traffic destined for a Web server will contain this port
ID. The Internet Assigned Numbers Authority (IANA) assigns
port numbers for common types of servers and functions.
7. If the LAN computer that you are making publicly available is
configured to use a non-standard port number for the type of
traffic it receives, type the non-standard port number in the
Local Port field.
This option translates the standard port number in packets
destined for your LAN computer to the non-standard number
you specify. For example, if your Web server uses (non-
standard) port 2000, but you expect incoming data packets to
refer to (standard) port 80, you would enter 2000 here (and
select HTTP or type 80 in the Destination Port fields). The
headers of incoming packets destined for port 80 will be
modified to refer to port 2000. The packet will then be routed
appropriately to the web server.
8. Follow steps 8-11 under "The NAPT rule" on page 125 to
submit your changes.