Chapter 11 Network Address Translation (NAT)
VMG3925-B10C/B30C User’s Guide
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11.4 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 VMG 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 VMG's WAN port
receives a response with a specific port number and protocol ("open" port), the VMG 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 82
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 VMG to record Jane’s computer IP address. The VMG
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 VMG 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 VMG
times out in three minutes with UDP (User Datagram Protocol) or two 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
VMG’s trigger port settings.
