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 address.
Trigger port forwarding allows 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 (\"open\" 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.
For example:
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 Zyxel Device to record Jane’s computer IP address. The Zyxel Device associates Jane's computer IP address with the "open" port range of 6970 – 7170.
3 The Real Audio server responds using a port number ranging between 6970 – 7170.
4 The Zyxel Device forwards the traffic to Jane’s computer IP address.
5 Only Jane can connect to the Real Audio server until the connection is closed or times out. The Zyxel Device times out in 3 minutes with UDP (User Datagram Protocol) or 2 hours with TCP/IP (Transfer Control Protocol/Internet Protocol).
Click Network Setting > NAT > Port Triggering to open the following screen. Use this screen to view your Zyxel Device’s trigger port settings.
*TCP port 7547 is reserved for system use.
*The sum of trigger ports in all rules must be less than 1000 and every open port range must be less than 1000. When the protocol is TCP/UDP, the ports are counted twice.
Network Setting > NAT > Port Triggering
The following table describes the labels in this screen.
Network Setting > NAT > Port Triggering 
Label
Description
Add New Rule
Click this to create a new rule.
#
This is the index number of the entry.
Status
This field displays whether the port triggering rule is active or not. A yellow bulb signifies that this rule is active. A gray bulb signifies that this rule is not active.
Service Name
This field displays the name of the service used by this rule.
WAN Interface
This field shows the WAN interface through which the service is forwarded.
Trigger Start Port
The trigger port is a port (or a range of ports) that causes (or triggers) the Zyxel Device to record the IP address of the LAN computer that sent the traffic to a server on the WAN.
This is the first port number that identifies a service.
Trigger End Port
This is the last port number that identifies a service.
Trigger Proto.
This is the trigger transport layer protocol.
Open Start Port
The open port is a port (or a range of ports) that a server on the WAN uses when it sends out a particular service. The Zyxel Device forwards the traffic with this port (or range of ports) to the client computer on the LAN that requested the service.
This is the first port number that identifies a service.
Open End Port
This is the last port number that identifies a service.
Open Protocol
This is the open transport layer protocol.
Modify
Click the Edit icon to edit this rule.
Click the Delete icon to delete an existing rule.
Add or Edit Port Triggering Rule
This screen lets you create new port triggering rules. Click Add New Rule in the Port Triggering screen or click a rule’s Edit icon to open the following screen. Use this screen to configure a port or range of ports and protocols for sending out requests and for receiving responses.
Network Setting > NAT > Port Triggering: Add or Edit
The following table describes the labels in this screen.
Network Setting > NAT > Port Triggering: Add or Edit 
Label
Description
Active
Click this switch to activate this rule.
Service Name
Enter a name to identify this rule. You can use up to 256 printable characters except [ " ], [ ` ], [ ' ], [ < ], [ > ], [ ^ ], [ $ ], [ | ], [ & ], or [ ; ]. Spaces are allowed.
WAN Interface
Select a WAN interface for which you want to configure port triggering rules.
Trigger Start Port
The trigger port is a port (or a range of ports) that causes (or triggers) the Zyxel Device to record the IP address of the LAN computer that sent the traffic to a server on the WAN.
Enter a port number or the starting port number in a range of port numbers.
Trigger End Port
Enter a port number or the ending port number in a range of port numbers.
Trigger Protocol
Select the transport layer protocol from TCP, UDP, or TCP/UDP.
Open Start Port
The open port is a port (or a range of ports) that a server on the WAN uses when it sends out a particular service. The Zyxel Device forwards the traffic with this port (or range of ports) to the client computer on the LAN that requested the service.
Enter a port number or the starting port number in a range of port numbers.
Open End Port
Enter a port number or the ending port number in a range of port numbers.
Open Protocol
Select the transport layer protocol from TCP, UDP, or TCP/UDP.
Cancel
Click Cancel to exit this screen without saving.
OK
Click OK to save your changes.