Figure 12-24 Trigger Port Forwarding Process: Example - ZyXEL Communications 10 User Manual

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 ZyWALL 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 ZyWALL's WAN
port receives a response with a specific port number and protocol ("incoming" port), the ZyWALL 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.
12.6.1 Trigger Port Forwarding Process
The following is an example of trigger port forwarding.

Figure 12-24 Trigger Port Forwarding Process: Example

1. Jane requests a file from the Real Audio server (port 7070).
2. Port 7070 is a "trigger" port and causes the ZyWALL to record Jane's computer IP address. The
ZyWALL associates Jane's computer IP address with the "incoming" port range of 6970-7170.
3. The Real Audio server responds using a port number ranging between 6970-7170.
4. The ZyWALL forwards the traffic to Jane's computer IP address.
NAT
ZyWALL 10~100 Series Internet Security Gateway
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