A network is a collection of computers and other devices that can send data to and receive data from each other, more or less in real time. A network is often connected by wires, and the bits of data are turned into electromagnetic waves that move through the wires. However, wireless networks transmit data through infrared light and microwaves , and many long-distance transmissions are now carried over fiber optic cables that send visible light through glass filaments. There's nothing sacred about any particular physical medium for the transmission of data. Theoretically, data could be transmitted by coal- powered computers that send smoke signals to each other. The response time (and environmental impact) of such a network would be rather poor.
Each machine on a network is called a node . Most nodes are computers, but printers, routers, bridges, gateways, dumb terminals, and Coca-Cola machines can also be nodes. You might use Java to interface with a Coke machine but otherwise , you'll mostly talk to other computers. Nodes that are fully functional computers are also called hosts . We will use the word node to refer to any device on the network, and the word host to refer to a node that is a general-purpose computer.
Every network node has an address , a series of bytes that uniquely identify it. You can think of this group of bytes as a number, but in general the number of bytes in an address or the ordering of those bytes (big endian or little endian) is not guaranteed to match any primitive numeric data type in Java. The more bytes there are in each address, the more addresses there are available and the more devices that can be connected to the network simultaneously .
Addresses are assigned differently on different kinds of networks. AppleTalk addresses are chosen randomly at startup by each host. The host then checks to see if any other machine on the network is using that address. If another machine is using the address, the host randomly chooses another, checks to see if that address is already in use, and so on until it gets one that isn't being used. Ethernet addresses are attached to the physical Ethernet hardware. Manufacturers of Ethernet hardware use pre-assigned manufacturer codes to make sure there are no conflicts between the addresses in their hardware and the addresses of other manufacturer's hardware. Each manufacturer is responsible for making sure it doesn't ship two Ethernet cards with the same address. Internet addresses are normally assigned to a computer by the organization that is responsible for it. However, the addresses that an organization is allowed to choose for its computers are assigned by the organization's Internet Service Provider (ISP). ISPs get their IP addresses from one of four regional Internet Registries (the registry for North America is ARIN, the American Registry for Internet Numbers, at http://www.arin.net/), which are in turn assigned IP addresses by the Internet Corporation for Assigned Names and Numbers (ICANN, at http://www.icann.org/).
On some kinds of networks, nodes also have names that help human beings identify them. At a set moment in time, a particular name normally refers to exactly one address. However, names are not locked to addresses. Names can change while addresses stay the same or addresses can change while the names stay the same. It is not uncommon for one address to have several names and it is possible, though somewhat less common, for one name to refer to several different addresses.
All modern computer networks are packet-switched networks: data traveling on the network is broken into chunks called packets and each packet is handled separately. Each packet contains information about who sent it and where it's going. The most important advantage of breaking data into individually addressed packets is that packets from many ongoing exchanges can travel on one wire, which makes it much cheaper to build a network: many computers can share the same wire without interfering. (In contrast, when you make a local telephone call within the same exchange, you have essentially reserved a wire from your phone to the phone of the person you're calling. When all the wires are in use, as sometimes happens during a major emergency or holiday, not everyone who picks up a phone will get a dial tone. If you stay on the line, you'll eventually get a dial tone when a line becomes free. In some countries with worse phone service than the United States, it's not uncommon to have to wait half an hour or more for a dial tone.) Another advantage of packets is that checksums can be used to detect whether a packet was damaged in transit.
We're still missing one important piece: some notion of what computers need to say to pass data back and forth. A protocol is a precise set of rules defining how computers communicate: the format of addresses, how data is split into packets, and so on. There are many different protocols defining different aspects of network communication. For example, the Hypertext Transfer Protocol (HTTP) defines how web browsers and servers communicate; at the other end of the spectrum, the IEEE 802.3 standard defines a protocol for how bits are encoded as electrical signals on a particular type of wire (among other protocols). Open, published protocol standards allow software and equipment from different vendors to communicate with each other: your web browser doesn't care whether any given server is a Unix workstation, a Windows box, or a Macintosh, because the server and the browser speak the same HTTP protocol regardless of platform.