User's Manual

Chapter 14 NAT
EMG2926-Q10A User’s Guide
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Figure 85 Multiple Servers Behind NAT Example
14.5.3 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 EMG2926-Q10A 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 EMG2926-Q10A's WAN port receives a response with a specific port number and protocol
("incoming" port), the EMG2926-Q10A 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.
14.5.4 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.
Figure 86 Trigger Port Forwarding Process: Example
1 Jane requests a file from the Real Audio server (port 7070).
Jane’s com puter
Rea l Audio Server
Port 7 0 7 0