ZyXEL Communications P-870HN-5xb User Manual page 152

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 requests a file from the Real Audio server (port 7070).
1
Port 7070 is a "trigger" port and causes the Device to record Jane's computer IP
2
address. The Device associates Jane's computer IP address with the "open" port
range of 6970-7170.
The Real Audio server responds using a port number ranging between 6970-7170.
3
The Device forwards the traffic to Jane's computer IP address.
4
Only Jane can connect to the Real Audio server until the connection is closed or
5
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
Jane's
Computer
Internet
Real Audio Server
P-870HN-5xb User's Guide