N600 Wireless Dual Band Router WNDR3400 User Manual
•The destination address is the IP address of www.example.com, which your computer finds by asking a DNS server.
•The destination port number is 80, the standard port number for a Web server process. Your computer then sends this request message to your router.
3.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 your router’s public IP address.
This 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 router, such as 33333. This is necessary because two computers could independently be using the same session number.
Your router 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 source address is the IP address of www.example.com.
•The source port number is 80, the standard port number for a Web server process.
•The destination address is the public IP address of your router.
•The destination port number is 33333.
The Web server then sends this reply message to your router.
5.Upon receiving the incoming message, your router checks its session table to determine whether there is an active session for port number 33333. Finding an active session, the router then modifies the message, restoring the original address information replaced by NAT. The message now contains the following address and port information:
•The source address is the IP address of www.example.com.
•The source port number is 80, the standard port number for a Web server process.
•The destination address is your computer’s IP address.
•The destination port number is 5678, the browser session that made the initial request.
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