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Chapter 10 Network Address Translation (NAT)
EMG5324-D10A User’s Guide
162
Port Forwarding
A port forwarding set is a list of inside (behind NAT on the LAN) servers, for example, web or FTP,
that you can make visible to the outside world even though NAT makes your whole inside network
appear as a single computer to the outside world.
Finding Out More
See Section 10.7 on page 168 for advanced technical information on NAT.
10.2 The Port Forwarding Screen
Use the Port Forwarding screen to forward incoming service requests to the server(s) on your
local network.
You may enter a single port number or a range of port numbers to be forwarded, and the local IP
address of the desired server. The port number identifies a service; for example, web service is on
port 80 and FTP on port 21. In some cases, such as for unknown services or where one server can
support more than one service (for example both FTP and web service), it might be better to
specify a range of port numbers. You can allocate a server IP address that corresponds to a port or
a range of ports.
The most often used port numbers and services are shown in Appendix E on page 353. Please refer
to RFC 1700 for further information about port numbers.
Note: Many residential broadband ISP accounts do not allow you to run any server
processes (such as a Web or FTP server) from your location. Your ISP may
periodically check for servers and may suspend your account if it discovers any
active services at your location. If you are unsure, refer to your ISP.
Configuring Servers Behind Port Forwarding (Example)
Let's say you want to assign ports 21-25 to one FTP, Telnet and SMTP server (A in the example),
port 80 to another (B in the example) and assign a default server IP address of 10.0.0.35 to a third
(C in the example). You assign the LAN IP addresses and the ISP assigns the WAN IP address. The
NAT network appears as a single host on the Internet.
Figure 71 Multiple Servers Behind NAT Example
A=10.0.0.33
D=10.0.0.36
C=10.0.0.35
B=10.0.0.34
WAN
LAN
10.0.0.1
IP Address assigned by ISP