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ZyWALL 2 Series User’s Guide
8-12 NAT
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.
For example:
Figure 8-7 Trigger Port Forwarding Example
1. A requests a file from the Real Audio server (port 7070).
2. Port 7070 is a “trigger” port and causes the ZyWALL to record A’s computer IP address. The ZyWALL
associates A’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 A’s computer IP address.
5. Only A can connect to the Real Audio server until the connection is closed or times out. The ZyWALL
times out in three minutes with UDP (User Datagram Protocol) or two hours with TCP/IP (Transfer Control
Protocol/Internet Protocol).
To change your ZyWALL’s trigger port settings, click SUA/NAT and the Trigger Port tab. The screen
appears as shown.