All mainstream communications among PCs is packet based. This means that a file (or any PC-PC conversation) is broken up into chunks called packets, which are usually about a thousand bytes long. Each packet is like a parcel traveling by UPS: It’s wrapped up securely and has an address plastered across it so anyone who looks at the package can know exactly where it’s going. A large device (say a bicycle) can be broken down into sections and the sections wrapped separately so that they’re small enough to travel through UPS’s system. So it is with networking. All the “boxes” are the same size, and anything too big to fit into one box is packed in as many boxes as it takes.
Once packed, UPS parcels are stamped with addresses and then loaded onto vehicles. Some of these are trucks, and some are trains. It doesn’t matter to the parcels—they get where they’re going regardless. Nor does it matter what type of road the vehicle travels over. It may be an asphalt road, or a railroad, or even a barge on a river or a canal. (UPS air costs extra!)
Nor does it matter (to the parcel) if the service transporting the package is UPS or Federal Express. The procedures that UPS follows are a little different from those at FedEx, but both companies get the parcels where they’re going.
This metaphor is one way to think about PC networking. Pieces of bicycles are packed into parcels. The parcels are presented at a UPS office for shipping. UPS carries them to their destination on some sort of vehicle. Data goes into packets. The packets are presented to a protocol for transport. The protocol carries them over some kind of physical link.
A protocol is pretty much what it sounds like: a set of rules and procedures for carrying data over a network. There are many networking protocols. Only a few are commonly used, and one of them, called Internet Protocol (IP) is by far the one used most widely today.
A physical link is also just what it sounds like: something in the physical world that carries data. Mostly you think of cables, but there are many different kinds of cables. Some are based on electrical signals passing over wires, but others contain glass or plastic fibers that carry data as pulses of light. It sounds odd, but you can have physical links based on infrared light traveling through the air, or radio waves. These don’t seem to be “physical,” but they’re out in the physical world and not inside the computer.
The magic in networking is that the protocol doesn’t care what physical link it’s using. A protocol works precisely the same way whether data is moving over wires, fiber-optic cable, infrared light beams, or radio waves. Your data gets there intact, whatever road it takes.
In virtually all cases these days, the protocol your PC will use is the Internet Protocol, which we’ll call IP from now on. Created along with the Internet in the late 1960s, IP has basically driven all other local area networking protocols out of the PC market. You’ll often hear the term TCP/IP (Transmission Control Protocol/Internet Protocol), which is the full name of the protocol used in most home networks and also the Internet. There are actually two protocols involved (TCP and IP), but they are so rarely used apart from one another that when most people say IP they really mean TCP/IP.
