Figure 186 Trigger Port Forwarding Process: Example - ZyXEL Communications Internet Security Appliance ZyWALL5UTM 4.0 User Manual

Internet security appliance

Table of Contents

Table 130 Port Forwarding

22.7 Port Triggering

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

Trigger port forwarding solves this problem by allowing computers on the LAN to

dynamically take turns using the service. The ZyWALL 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 ZyWALL's WAN port receives a response with a specific

port number and protocol ("incoming" port), the ZyWALL 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 186 Trigger Port Forwarding Process: Example

1 Jane requests a file from the Real Audio server (port 7070).

2 Port 7070 is a "trigger" port and causes the ZyWALL to record Jane's computer IP

address. The ZyWALL associates Jane's computer IP address with the "incoming" port

range of 6970-7170.

3 The Real Audio server responds using a port number ranging between 6970-7170.

Chapter 22 Network Address Translation (NAT)

Enter the inside IP address of the server here.

Click Apply to save your changes back to the ZyWALL.

Click Reset to begin configuring this screen afresh.

ZyWALL 5/35/70 Series User's Guide