Advanced Settings
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Wireless Cable Gateway CG3300CMR
A typical application of port forwarding can be shown by reversing the client-server
relationship from the previous web server example. In this case, a browser on a remote
computer accesses a web server running on a computer in your local network. Using port
forwarding, you can tell the gateway, “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.
The 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 gateway. 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
gateway.
Destination port number
. 80, which is the standard port number for a web server
process.
The remote computer then sends this request message through the Internet to your
gateway.
2.
Your gateway 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 is forwarded to local IP address 192.168.1.123. Therefore, your
gateway modifies the destination information in the request message:
The destination address is replaced with 192.168.1.123.
Your gateway then sends this request message to your local network.
3.
Your 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
gateway.
4.
Your gateway 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.
To configure port forwarding, you need to know which inbound ports the application needs.
Usually you can determine this information by contacting the publisher of the application or
the relevant user groups and newsgroups.
