D-Link DI-206 - Manual (Page 108)

DI-206 ISDN Remote Router
96 Configuration and
Management
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Port – This is a destination port number used by TCP and UDP to
de-multiplex incoming IP packets.
In the above example, incoming packets with the global destination IP
Address (211.11.22.3) and global destination TCP/UDP port (21) will
be translated to a packet with the local destination IP Address (1.1.1.5)
and local TCP/UDP port (21).
Port 21 is assigned to FTP servers. Please see Appendix D for more
commonly assigned port numbers, or RFC 1700 for a more complete
list.
Configure NAPT for Special Aps
Some applications programs that are used over the Internet such as
Microsoft NetMeeting, Diablo, CU See Me and Xwindows send
information to a certain port number or within a specified range of port
numbers. The exact port number used is specific to the application.
However, if you find that you are having trouble using an application
over the Internet and you are using NAPT, you may need to exempt
certain port numbers from the NAPT port translation process. Please
refer to the user guide for the program to find out whether it transmits
and receives data only through specified IP port numbers. In order for
these programs to work with NAPT, the IP port numbers required by
these applications must be entered in the Configure NAPT for Special
APs screen shown below.