Trigger Port Forwarding Example; Two Points To Remember About Trigger Ports; Figure 70 Trigger Port Forwarding Process: Example - ZyXEL Communications P-334WT User Manual

P-334WT User's Guide
Trigger port forwarding solves this problem by allowing computers on the LAN to
dynamically take turns using the service. The Prestige 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 Prestige's WAN port receives a response with a specific
port number and protocol ("incoming" port), the Prestige 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.

8.6.1 Trigger Port Forwarding Example

The following is an example of trigger port forwarding.

Figure 70 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 Prestige to record Jane's computer IP address.
The Prestige 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 Prestige forwards the traffic to Jane's computer IP address.
5 Only Jane can connect to the Real Audio server until the connection is closed or times
out. The Prestige times out in three minutes with UDP (User Datagram Protocol), or two
hours with TCP/IP (Transfer Control Protocol/Internet Protocol).

8.6.2 Two Points To Remember About Trigger Ports

1 Trigger events only happen on data that is going coming from inside the Prestige and
going to the outside.
2 If an application needs a continuous data stream, that port (range) will be tied up so that
another computer on the LAN can't trigger it.
144 Chapter 8 Network Address Translation (NAT)