Netgear DGN100Bv3 Benutzerhandbuch

By default, the modem router blocks inbound traffic from the Internet to your computers 
except replies to your outbound traffic. You might need to create exceptions to this rule for 
these purposes:

To enable remote computers on the Internet to access a server on your local network. 

To enable certain applications and games to work correctly if the modem router does not 
recognize their replies.

Your modem router provides two features for creating these exceptions: port forwarding and 
port triggering. The next sections provide background information to help you understand 
how port forwarding and port triggering work, and the differences between the two.

When a computer on your network needs to access a computer on the Internet, your 
computer sends your modem router a message containing the source and destination 
address and process information. Before forwarding your message to the remote computer, 
your modem router has to modify the source information and create and track the 
communication session so that replies can be routed back to your computer. 

Here is an example of normal outbound traffic and the resulting inbound responses:

You open a browser, and your operating system assigns port number 5678 to this 
browser session. 

You type http://www.example.com into the URL field, and your computer creates a web page 
request message with the following address and port information. The request message is 
sent to your modem router:

Source address. Your computer’s IP address

Source port number. 5678, which is the browser session

Destination address. The IP address of www.example.com, which your computer 
finds by asking a DNS server

Destination port number. 80, which is the standard port number for a web server 
process

Your modem router 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 modem router 
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 your modem router’s public IP address. This 
requirement 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 chosen by the modem router, such 
as 33333. This requirement is necessary because two computers might 
independently be using the same session number.