D-Link DI-308 - Manual (Page 74)

DI-308 ISDN Remote Router
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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 Ap[plication]s
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.