Trigger Port Forwarding; Trigger Port Forwarding Example - ZyXEL Communications EMG3425-Q10A User Manual

12.5.3 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 EMG3425-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 EMG3425-Q10A's WAN port receives a response with a specific port number and protocol
("incoming" port), the EMG3425-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.

12.5.4 Trigger Port Forwarding Example

The following is an example of trigger port forwarding.
Figure 62 Trigger Port Forwarding Process: Example
Jane's computer
Jane requests a file from the Real Audio server (port 7070).
1
Port 7070 is a "trigger" port and causes the EMG3425-Q10A to record Jane's computer IP address.
2
The EMG3425-Q10A 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.
3
The EMG3425-Q10A forwards the traffic to Jane's computer IP address.
4
Only Jane can connect to the Real Audio server until the connection is closed or times out. The
5
EMG3425-Q10A times out in three minutes with UDP (User Datagram Protocol), or two hours with
TCP/IP (Transfer Control Protocol/Internet Protocol).
Chapter 12 NAT
EMG3425-Q10A User's Guide
107
Real Audio Server
Port 7070