The Port Triggering Screen - ZyXEL Communications PMG2006-T20A User Manual

9.4 The Port Triggering Screen

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 PMG2006-T20A 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
PMG2006-T20A's WAN port receives a response with a specific port number and protocol ("open" port),
the PMG2006-T20A 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.
For example:
Figure 42 Trigger Port Forwarding Process: Example
Jane requests a file from the Real Audio server (port 7070).
1
Port 7070 is a "trigger" port and causes the PMG2006-T20A to record Jane's computer IP address. The
2
PMG2006-T20A associates Jane's computer IP address with the "open" port range of 6970-7170.
The Real Audio server responds using a port number ranging between 6970-7170.
3
The PMG2006-T20A 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
PMG2006-T20A times out in three minutes with UDP (User Datagram Protocol) or two hours with TCP/IP
(Transfer Control Protocol/Internet Protocol).
Click Network Setting > NAT > Port Triggering to open the following screen. Use this screen to view your
PMG2006-T20A's trigger port settings.
Chapter 9 Network Address Translation (NAT)
PMG2006-T20A User's Guide
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