Chapter 11 Network Address Translation (NAT)
EMG6726/8726-B10A User’s Guide
154
NAT
In the simplest form, NAT changes the source IP address in a packet received from a subscriber (the
inside local address) to another (the inside global address) before forwarding the packet to the WAN
side. When the response comes back, NAT translates the destination address (the inside global address)
back to the inside local address before forwarding it to the original inside host.
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
for advanced technical information on NAT.
11.2 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
. 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 192.168.1.35 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.
