Chapter 8 NAT
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 ZyXEL Device 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 ZyXEL Device's WAN port receives a response with a specific port number and protocol ("incoming" port), the ZyXEL Device 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.
8.1.2.1 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.
Figure 68 Trigger Port Forwarding Process: Example
1Jane requests a file from the Real Audio server (port 7070).
2Port 7070 is a “trigger” port and causes the ZyXEL Device to record Jane’s computer IP address. The ZyXEL Device associates Jane's computer IP address with the "incoming" port range of
3The Real Audio server responds using a port number ranging between
4The ZyXEL Device forwards the traffic to Jane’s computer IP address.
5Only Jane can connect to the Real Audio server until the connection is closed or times out. The ZyXEL Device times out in three minutes with UDP (User Datagram Protocol), or two hours with TCP/IP (Transfer Control Protocol/Internet Protocol).
8.1.2.2Two Points To Remember About Trigger Ports
1Trigger events only happen on data that is going coming from inside the ZyXEL Device and going to the outside.
2If 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.
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