Nat Port Forwarding: Services And Port Numbers; Nat Port Forwarding Example; Trigger Port Forwarding - ZyXEL Communications NR2101 User Manual

8.3.1 NAT Port Forwarding: Services and Port Numbers

A port forwarding set is a list of inside (behind NAT on the LAN) servers, for example, web or FTP, that you
can make accessible to the outside world even though NAT makes your whole inside network appear
as a single machine to the outside world.
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.
In addition to the servers for specified services, NAT supports a default server. A service request that does
not have a server explicitly designated for it is forwarded to the default server. If the default is not
defined, the service request is simply discarded.
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.

8.3.2 NAT 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.
Figure 88 Multiple Servers Behind NAT Example

8.3.3 Trigger Port Forwarding

Some services use a dedicated range of ports on the client side and a dedicated range of ports on the
server side. With regular port forwarding you set a forwarding port in NAT to forward a service (coming in
from the server on the WAN) to the IP address of a computer on the client side (LAN). The problem is that
port forwarding only forwards a service to a single LAN IP address. In order to use the same service on a
Chapter 8 NAT
NR2101 User's Guide
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