Figure 7-7 Trigger Port Forwarding Process: Example - Nortel Contivity 221 User Manual

Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically
take turns using the service. The Contivity 221 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 Contivity 221's WAN port receives a response with a specific port number and
protocol ("incoming" port), the Contivity 221 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.
7.6.1 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.

Figure 7-7 Trigger Port Forwarding Process: Example

1. Jane (A) requests a file from the Real Audio server (port 7070).
2. Port 7070 is a "trigger" port and causes the Contivity 221 to record Jane's computer IP address.
The Contivity 221 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.
4. The Contivity 221 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
Contivity 221 times out in three minutes with UDP (User Datagram Protocol) or two hours with
TCP/IP (Transfer Control Protocol/Internet Protocol).
7.6.2 Two Points To Remember About Trigger Ports
1. Trigger events only happen on data that is going coming from inside the Contivity 221 and
going to the outside.
Contivity 221 VPN Switch User's Guide
NAT 7-13