As discussed in the previous articles, every computer connected to the Internet has a unique address. Let's say your IP address is 1.2.3.4 and you want to send a message to the computer with the IP address 5.6.7.8. The message you want to send is "Hello computer 5.6.7.8!” Let's say you've dialed into your ISP from home and the message must be transmitted over the phone line. Therefore the message must be translated from alphabetic text into electronic signals, transmitted over the Internet, and then translated back into alphabetic text.


How is this accomplished? Through the use of a protocol stack. Every computer needs one to communicate on the Internet and it is usually built into the computer's operating system (i.e. Windows, UNIX, etc.). The protocol stack used on the Internet is referred to as the TCP/IP protocol stack

 If we were to follow the path that the message "Hello computer 5.6.7.8!" took from our computer to the computer with IP address 5.6.7.8, it would happen something like this:
  • The message would start at the top of the protocol stack on your computer and work it's way downward.
  • If the message to be sent is long, each stack layer that the message passes through may break the message up into smaller chunks of data. This is because data sent over the Internet (and most computer networks) are sent in manageable chunks. On the Internet, these chunks of data are known as packets.
  • The packets would go through the Application Layer and continue to the TCP layer. Each packet is assigned a port number, which is used by program on the destination computer to receive the message because it will be listening on a specific port.
  • After going through the TCP layer, the packets proceed to the IP layer. This is where each packet receives its destination address, 5.6.7.8.
  • Now that our message packets have a port number and an IP address, they are ready to be sent over the Internet. The hardware layer takes care of turning our packets containing the alphabetic text of our message into electronic signals and transmitting them over the phone line.
  • On the other end of the phone line your ISP has a direct connection to the Internet. The ISPs router examines the destination address in each packet and determines where to send it. Often, the packet's next stop is another router. More on routers and Internet infrastructure later.
  • Eventually, the packets reach computer 5.6.7.8. Here, the packets start at the bottom of the destination computer's TCP/IP stack and work upwards.
  • As the packets go upwards through the stack, all routing data that the sending computer's stack added (such as IP address and port number) is stripped from the packets.
  • When the data reaches the top of the stack, the packets have been re-assembled into their original form, "Hello computer 5.6.7.8!"