Advanced Settings
103
N600 Wireless Dual Band Gigabit ADSL2+ Modem Router DGND3700v2
Note: Only one computer at a time can use the triggered application.
Port Forwarding to Permit External Host Communications
In both of the preceding examples, your computer initiates an application session with a
server computer on the Internet. However, you might need to allow a client computer on the
Internet to initiate a connection to a server computer on your network. Normally, your wireless
modem router ignores any inbound traffic that is not a response to your own outbound traffic.
You can configure exceptions to this default rule by using the port forwarding feature.
A typical application of port forwarding can be shown by reversing the client-server
re
lationship from the previous web server example. In this case, a remote computer’s
browser needs to access a web server running on a computer in your local network. Using
port forwarding, you can tell the wireless modem router, “When you receive incoming traffic
on port 80 (the standard port number for a web server process), forward it to the local
computer at 192.168.1.123.” The following sequence shows the effects of the port forwarding
rule you have defined:
1. T
he user of a remote computer opens a browser and requests a web page from
www.example.com, which resolves to the public IP address of your wireless modem
router. The remote computer composes a web page request message with the following
destination information:
Destination address.
The IP address of www.example.com, which is the address of your
wireless modem router.
Destination port number. 8
0, which is the standard port number for a web server
process.
The remote computer then sends this request message through the Internet to your
wireless modem route
r.
2. Y
our wireless modem router receives the request message and looks in its rules table for
any rules covering the disposition of incoming port 80 traffic. Your port forwarding rule
specifies that incoming port 80 traffic should be forwarded to local IP address
192.168.1.123. Therefore, your wireless modem router modifies the destination information
in the request message:
The destination address is replaced with 192.168.1.123.
Your wireless modem router then sends this request message to your local network.
3. Y
our web server at 192.168.1.123 receives the request and composes a return message
with the requested web page data. Your web server then sends this reply message to your
wireless modem router.
4. Y
our wireless modem router performs Network Address Translation (NAT) on the source IP
address, and sends this request message through the Internet to the remote computer,
which displays the web page from www.example.com.
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