ZyXEL Communications NR2301 - Trigger Port Forwarding; Trigger Port Forwarding Example

Chapter 9 Device Management
NR2301 User’s Guide
99
9.16.3 Trigger Port Forwarding
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 NR2301 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 NR2301's WAN
port receives a response with a specific port number and protocol ("incoming" port), the NR2301
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.
9.16.4 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.
Figure 65 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 NR2301 to record Jane’s computer IP address. The NR2301
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 NR2301 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 NR2301
times out in three minutes with UDP (User Datagram Protocol), or two hours with TCP/IP (Transfer Control
Protocol/Internet Protocol).