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10.6.2 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 62 Multiple Servers Behind NAT Example
10.6.3 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
different LAN computer, you have to manually replace the LAN computer's IP address in the forwarding
port with another LAN computer's IP address.
Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically take turns
using the service. The Zyxel Device records the IP address of a LAN computer that sends traffic to the
WAN to request a service with a specific port number and protocol (a "trigger" port). When the Zyxel
Device's WAN port receives a response with a specific port number and protocol ("incoming" port), the
Zyxel Device forwards the traffic to the LAN IP address of the computer that sent the request. After that
computer's connection for that service closes, another computer on the LAN can use the service in the
same manner. This way you do not need to configure a new IP address each time you want a different
LAN computer to use the application.
10.6.4 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.
Chapter 10 NAT
LTE3301-M209 User's Guide
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