The Trigger Port Screen - ZyXEL Communications P-870H Series User Manual

Chapter 8 Network Address Translation (NAT)

8.5 The Trigger Port Screen

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 ("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 67 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
124
Jane's
Computer
Internet
Real Audio Server
P-870H/HW Series User's Guide