ZyXEL Communications P-870HN-51B - V1.0 Manual page 138

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 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 ("open"
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.
For example:
Figure 69 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 ZyXEL Device to record Jane's
2
computer IP address. The ZyXEL 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 ZyXEL 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 ZyXEL Device times out in three minutes with UDP (User Datagram
Protocol) or two hours with TCP/IP (Transfer Control Protocol/Internet Protocol).
138
Jane's
Computer
Internet
Real Audio Server
P-870HN-51b User's Guide