Trigger Port Forwarding; Trigger Port Forwarding Example - ZyXEL Communications NBG4615 User Manual

Chapter 20 NAT

20.5.3 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 NBG4615 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 NBG4615's WAN port receives a response with a specific port number and protocol ("incoming"
port), the NBG4615 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.

20.5.4 Trigger Port Forwarding Example

The following is an example of trigger port forwarding.
Figure 115 Trigger Port Forwarding Process: Example
Jane's computer
Jane requests a file from the Real Audio server (port 7070).
1
Port 7070 is a "trigger" port and causes the NBG4615 to record Jane's computer IP address. The
2
NBG4615 associates Jane's computer IP address with the "incoming" port range of 6970-7170.
The Real Audio server responds using a port number ranging between 6970-7170.
3
The NBG4615 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 times out. The
5
NBG4615 times out in three minutes with UDP (User Datagram Protocol), or two hours with TCP/IP
(Transfer Control Protocol/Internet Protocol).
178
Real Audio Server
Port 7070
NBG4615 User's Guide