ZyXEL Communications WAH7608 - Trigger Port Forwarding; Trigger Port Forwarding Example

Chapter 11 NAT
WAH7608 User’s Guide
87
Figure 49 Multiple Servers Behind NAT Example
11.4.3 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 WAH7608 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 WAH7608's
WAN port receives a response with a specific port number and protocol ("incoming" port), the WAH7608
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.
11.4.4 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.
Figure 50 Trigger Port Forwarding Process: Example
1 Jane requests a file from the Real Audio server (port 7070).