Port Triggering To Open Incoming Ports - NETGEAR CG3700EMR-1CMNDS User Manual

Wireless Cable Voice Gateway CG3700EMR-1CMNDS
• Destination port number. 80, which is the standard port number for a web server
process.
3. Your gateway creates an entry in its internal session table describing this communication
session between your computer and the web server at www.example.com. Before sending
the web page request message to www.example.com, your gateway stores the original
information and then modifies the source information in the request message, performing
Network Address Translation (NAT):
• The source address is replaced with the public IP address of your gateway. This step
is necessary because your computer uses a private IP address that is not globally
unique and cannot be used on the Internet.
• The source port number is changed to a number that is chosen by the gateway, such
as 33333. This step is necessary because two computers could independently be
using the same session number.
Your gateway then sends this request message through the Internet to the web server at
www.example.com.
4. The web server at www.example.com composes a return message with the requested web
page data. The return message contains the following address and port information. The
web server then sends this reply message to your gateway:
• Source address. The IP address of www.example.com.
• Source port number. 80, which is the standard port number for a web server
process.
• Destination address. The public IP address of your gateway.
• Destination port number. 33333.
5. When your gateway receives the incoming message, it checks its session table for an active
session for port number 33333. Finding an active session, the gateway then modifies the
message to restore the original address information that is replaced by NAT. Your gateway
sends this reply message to your computer, which displays the web page from
www.example.com. The message now contains the following address and port information:
• Source address. The IP address of www.example.com.
• Source port number. 80, which is the standard port number for a web server
process.
• Destination address. The IP address of your computer.
• Destination port number. 5678, which is the browser session that made the initial
request.
6. When you finish your browser session, your gateway eventually detects a period of inactivity
in the communications. Your gateway then removes the session information from its session
table, and incoming traffic is no longer accepted on port number 33333.

Port Triggering to Open Incoming Ports

In the preceding example, requests are sent to a remote computer by your gateway from a
particular service port number, and replies from the remote computer to your gateway are
directed to that port number. If the remote server sends a reply to a different port number,
your gateway does not recognize it and discards it. However, some application servers (such
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