Every computer connected to Internet has a unique address. Let's just say your IP address is 1.2.3.4 and you want to send a message to computer with IP address 5.6.7.8. Message you want to send is "Hello computer 5.6.7.8!" Let's say you've dialed into your ISP from home and message should be transmitted over phone line. Hence the message should be translated from alphabetic text into electronic signals, transmitted over Internet, and then translated back into alphabetic text. How is this achieved? By the use of a protocol stack. Each computer needs one to communicate on Internet and it is generally built into computer's operating system (i.e. Unix, Windows, etc.).