User`s guide

Chapter 13 Network Address Translation (NAT) Screens
LAN-Cell 2 User’s Guide
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13.5 Port Triggering
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 LAN-Cell 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 LAN-Cell's WAN port receives a response with a
specific port number and protocol ("incoming" port), the LAN-Cell 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 179 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 LAN-Cell to record Jane’s computer IP
address. The LAN-Cell 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 LAN-Cell 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 LAN-Cell times out in three minutes with UDP (User Datagram Protocol) or
two hours with TCP/IP (Transfer Control Protocol/Internet Protocol).
Apply Click Apply to save your changes back to the LAN-Cell.
Reset Click Reset to begin configuring this screen afresh.
Table 110 ADVANCED > NAT > Port Forwarding
LABEL DESCRIPTION