58 | Chapter 5: Fine-Tuning Your Network
N150 Wireless Router User Manual
How Your Computer Accesses a Remote Computer through Your Router
When a computer on your network needs to access a computer on the Internet, your
computer sends your router a message containing source and destination address and
process information. Before forwarding your message to the remote computer, your router
must modify the source information and must 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:
1. You open Internet Explorer, beginning a browser session on your computer. Invisible to
you, your operating system assigns a service number (port number) to every
communication process running on your computer. In this example, let’s say Windows
assigns port number 5678 to this browser session.
2. You ask your browser to get a Web page from the Web server at www.example.com. Your
computer composes a Web page request message with the following address and
port information:
• The source address is your computer’s IP address.
• The source port number is 5678, the browser session.
• 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.