Every computer connected to the Internet (even a beat-up old Apple II) has a unique address: a number whose format is defined by the Internet Protocol (IP), the standard that defines how messages are passed from one machine to another on the Net. An IP address is made up of four numbers, each less than 256, joined together by periods, such as 18.104.22.168 and 22.214.171.124.
While computers deal only with numbers, people prefer names. For this reason, most computers also have names bestowed upon them. By current estimates, there are hundreds of millions, if not billions, of devices on the Net, so it would be very difficult to come up with that many unique names, let alone keep track of them all. Instead, the Internet is a network of networks, and is divided into groups known as domains, which are further divided into one or more subdomains. So, while you might choose a very common name for your computer, it becomes unique when you append, like surnames, all of the machine’s domain names as a period-separated suffix, creating a fully qualified domain name.
This naming stuff is easier than it sounds. For example, the fully qualified domain name www.oreilly.com translates to a machine named “www” that’s part of the domain known as “oreilly,” which, in turn, is part of the commercial (com) branch of the Internet. Other branches of the Internet include educational institutions (edu), nonprofit organizations (org), the U.S. government (gov), and Internet service providers (net). Computers and networks outside the United States may have two-letter abbreviations at the end of their names: for example, “ca” for Canada, “jp” for Japan, and “uk” for the United Kingdom.
Special computers, known as nameservers, keep tables of machine names and associated IP addresses and translate one into the other for us and for our machines. Domain names must be registered and paid for through any one of the now many for-profit registrars.[*] Once a unique domain name is registered, its owner makes it and its address available to other domain nameservers around the world.