Chapter 10 Network Address Translation (NAT)
VMG8924-B10A and VMG8924-B30A Series User’s Guide
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10.4 The Port Triggering Screen
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 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
Device's WAN port receives a response with a specific port number and protocol ("open" port), the
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:
Figure 86 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 Device to record Jane’s computer IP address. The
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 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
Device times out in three minutes with UDP (User Datagram Protocol) or two hours with TCP/IP
(Transfer Control Protocol/Internet Protocol).
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