Trigger Port Forwarding Example; Two Points To Remember About Trigger Ports - ZyXEL Communications NBG-5715 User Manual

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 NBG5715 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 NBG5715's WAN port receives a response with a specific port number and protocol ("incoming"
port), the NBG5715 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 67 Trigger Port Forwarding Process: Example
Jane requests a file from the Real Audio server (port 7070).
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Port 7070 is a "trigger" port and causes the NBG5715 to record Jane's computer IP address. The
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NBG5715 associates Jane's computer IP address with the "incoming" port range of 6970-7170.
The Real Audio server responds using a port number ranging between 6970-7170.
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The NBG5715 forwards the traffic to Jane's computer IP address.
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Only Jane can connect to the Real Audio server until the connection is closed or times out. The
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NBG5715 times out in three minutes with UDP (User Datagram Protocol), or two hours with TCP/IP
(Transfer Control Protocol/Internet Protocol).

14.5.5 Two Points To Remember About Trigger Ports

Trigger events only happen on data that is going coming from inside the NBG5715 and going to the
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outside.
If an application needs a continuous data stream, that port (range) will be tied up so that another
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computer on the LAN can't trigger it.
NBG5715 User's Guide
Chapter 14 NAT
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