Thirty years ago, a computer network was composed of a single large machine connected by long sets of wires to a number of small green-screen terminals. The terminals were only capable of displaying twenty-three lines of text using a restricted character set. The most exciting form of communication that took place over the computer network was the Unix utility called ``talk'', which permitted two users of the same computer to type text messages to each other.
Clearly, times have changed. No one can work in science or engineering today without a computer nearby. Electronic mail, typesetting, and web services accelerate the flow of information to speeds unimaginable just a few decades ago. A variety of data compression schemes, coupled with large-bandwidth networks, make transferring images just as easy as transferring text. Pocket telephones today have more power and memory than that mainframe that supported 100 green-screen terminals in 1980.
What is the Internet? The name itself says much about it: it is a network that connects networks. In other words, while one may have a local network anywhere -- in the office, in the home -- that network is a world unto itself unless it is connected to other networks. The Internet comprises the infrastructure and software that permits you to connect from your local network to machines all over the world. The infrastructure is composed of cables, network devices that switch and route the signals to the next station, and the collection of languages and programs that provide the protocols for these devices to talk to each other.
There are many kinds of media used to create computer networks, from small cables made of copper to fiber optic strands. To some extent, the properties of the media determine the types of packets that are sent across them, but different packet technologies may use similar media. For example, fiber optic cables are used for FDDI packets on some networks, for ATM transmissions on others, and for fast Ethernet packets on others still. The key to the Internet is that all of these different media, transmitting different kinds of packets, may still communicate through the use of standardized protocols, such as TCP/IP, and at higher levels, FTP and SSH.