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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.
Trigger events only happen on outgoing data (from the ZyXEL device).
Only one LAN computer can use a trigger port (range) at a time. Therefore, if 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 until that stream finishes.
9.4.1 Configuring Trigger Port
Select Full Feature in, click Edit and then click the Trigger Port tab to bring up the next
screen.
Chapter 9 Configuration > NAT
Vantage CNM 2.0 User's Guide
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