Trigger Port Forwarding - ZyXEL Communications NBG-417N - V1.0 Manual

address of 192.168.1.35 to a third (C in the example). You assign the LAN IP
addresses and the ISP assigns the WAN IP address. The NAT network appears as a
single host on the Internet.
Figure 70 Multiple Servers Behind NAT Example

11.9 Trigger Port Forwarding

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 NBG-417N 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 NBG-417N's WAN port
receives a response with a specific port number and protocol ("incoming" port),
the NBG-417N 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.
NBG-417N User's Guide
Chapter 11 Network Address Translation (NAT)
NBG-417N
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