ZyXEL P-870HN-51D User Guide - Page 152
Trigger Port Forwarding Process: Example, Jane's, Computer, Real Audio Server,
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Chapter 8 Network Address Translation (NAT) Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically take turns using the service. The 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 Device's WAN port receives a response with a specific port number and protocol ("open" port), the 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. For example: Figure 73 Trigger Port Forwarding Process: Example Jane's Computer Internet Real Audio Server 1 Jane requests a file from the Real Audio server (port 7070). 2 Port 7070 is a "trigger" port and causes the Device to record Jane's computer IP address. The Device associates Jane's computer IP address with the "open" port range of 6970-7170. 3 The Real Audio server responds using a port number ranging between 6970-7170. 4 The Device 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 Device times out in three minutes with UDP (User Datagram Protocol) or two hours with TCP/IP (Transfer Control Protocol/Internet Protocol). 152 P-870HN-5xb User's Guide