Verizon MBR1515L 用户手册

Port Forwarding/Port Triggering

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

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

To allow certain applications and games to work correctly when your router does not 
recognize their replies.

Your 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.

Remote Computer Access Basics

When a computer on your network accesses a computer on the Internet, your computer 
sends your router a message containing the source and destination address and process 
information. Before forwarding your message to the remote computer, your 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 router.

Source address. The IP address of your computer.

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 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 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 the public IP address of your router. 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 router, such as 
33333. This step is necessary because two computers could independently be using 
the same session number.