ZyXEL Communications NBG-510S, User Manual

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Chapter 11 NAT and Firewall (WAN to LAN)
NBG-510S User’s Guide
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11.5 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 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 ("incoming" 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.
11.5.1 Trigger Port Forwarding Example
The following is an example of trigger port forwarding.
Figure 60 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 "incoming"
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 three minutes with UDP (User Datagram Protocol),
or two hours with TCP/IP (Transfer Control Protocol/Internet Protocol).
11.5.2 Two Points To Remember About Trigger Ports
1 Trigger events only happen on data that is going coming from inside the ZyXEL Device
and going to the outside.