Application Layer - The programs you use to access a network (for example, web browsers such as Internet Explorer and e-mail clients such as Microsoft Outlook).
Transport Control Protocol (TCP) Layer - This is the part of your system that breaks your message into packets, addresses and numbers them, and adds error-checking features. Also, when you receive a message, the TCP layer places them in order (if they arrived out of order), checks for errors, and requests retransmission of missing or erroneous packets.
Internet Protocol (IP) - This is the part of your system that maintains your IP address. With most Internet Service Providers (ISPs), you usually get a different IP address each time you connect to the network (think of it as a "loaner"). Your IP layer also attaches information to each packet you send that can be used by routers on the Internet to get your packet to its ultimate destination.
Local Network Access Protocol (NAP) Layer - This is the part of your system that is concerned with how you communicate with your local network (e.g., your ISP). For instance, your NAP layer may tell your gateway that you wish to send a message to such-and-such address on the Internet.
Physical Layer - This is your physical connection to your local network. You may connect with a Network Interface Card (NIC) or with a modem. Software in your NIC or modem manages the speed of transmission and other elements of how you communicate with the rest of the network.
For example, suppose you want to send your friend a copy of a PowerPoint presentation you have made. You might use Outlook (Application layer) to send an e-mail with the presentation attached. Your personal computer's operating system (e.g., Windows 98) will break the message down into packets and make sure they have the correct address and other information they need to traverse the Internet (IP and TCP layers). Your operating system also manages how your computer communicates with your ISP (Local Network Access layer), for instance, B.U. Finally, your Ethernet card (Physical layer) might actually send the message along wires in your dorm to one of B.U.'s gateways.
The gateway's network hardware (Physical layer) receives your e-mail. The gateway has software that manages network traffic on B.U.'s local networks (Local Network Access layer). It also has software that knows how to route your e-mail onto the Internet (IP layer). Once on the Internet, your e-mail is sent from router to router until it arrives at your friend's ISP. At the point, the process is for the most part reversed until your friend sees your message and attachment in her e-mail client.
Why all the layers? Each layer represents a logical task to be accomplished, and usually is represented by a process running on your machine. It might be possible to have just one layer, but that would mean that you would have one piece of software that would manage all aspects of your network connection, from providing you with windows in which to edit your e-mail to putting bits onto the network cable running from the back of your computer. Instead, however, tasks are broken down for reasons of modularity, reliability, and efficiency.
Modularity means that you can easily replace a particular layer. For instance, if you get a new network card, you simply install it, but you do not have to reconfigure all of the other layers. Also, you can run different applications (web browsers, e-mail, ftp clients, Napter) without reconfiguring all the other layers. Finally, if you have a notebook computer, you could plug it into different networks at different places, and you need only reconfigure the layers that bear on the particular network connections.
Reliability comes from the fact that each layer is independent of the others. If there is a problem with one layer, for instance, your web browser, other applications still can use the other layers.
Efficiency comes from the fact that tasks can be interleaved with an operating system that supports multiprogramming (most modern ones do). This means that when you click "send" on your e-mail program, the processes which take care of the TCP, IP, Network Access, and physical layers can be processing the message while you are already reading your next message. Multiprogramming turns out to be critical in improving the performance of gateways and other computer systems that manage more than one user.