Trigger Port Forwarding; Trigger Port Forwarding Example - ZyXEL Communications NBG-418N v2 User Manual

Figure 75 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 NBG-418N v2 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 NBG-418N v2's WAN port receives a response with a specific port number and protocol
("incoming" port), the NBG-418N v2 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.
Figure 76 Trigger Port Forwarding Process: Example
Jane's computer
Jane requests a file from the Real Audio server (port 7070).
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NBG-418N v2 User's Guide
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Real Audio Server
Port 7070