Domain Names 101 (continued)

Names to Numbers

Under the covers, the Internet doesn't really work using names. The key to connecting all the computers on the Internet are numeric values called Internet Protocol (IP) addresses. An IP address consists of four numbers, separated by periods, such as (each number can range from 0 to 255). Every computer that is directly connected on the Internet has a unique IP address, just as every directly connected telephone has its own phone number.

Of course, not every phone in the telephone system is directly connected -- some are connected to corporate switchboards that you must call first to get to an actual telephone at somebody's desk. In the same way, not every computer on the Internet is directly connected -- some are connected to corporate firewalls that you must pass through to get to an actual computer. For the most part, domain names are concerned with computers that are not behind a firewall, because these computers are usually the ones accessible to the general public. The domain name system focuses on computers that are directly connected, and that deliver some kind of service -- Web publishing, e-mail services, or file transfer -- to the general public.

The job of the Domain Name System is to tie easy-to-remember name-based addresses to IP addresses, a process known as name resolution. Names are resolved into IP addresses using a special kind of Internet server called a name server. A name server is like any other Internet server, such as a Web or mail server, except that it's dedicated to the task of translating names to numbers. The translation is a two-step process and involves as many as three different name servers.

All About Servers

All the second-level names on the entire Internet are stored in a giant complex of government-run name servers called the root name servers. When you type a Web address into your Web browser and hit enter, your browser asks a local name server, usually operated by your ISP, to resolve the name. There's a chance that the domain name you are trying to reach is served or cached on the local name server, and if that's the case, the server will respond directly with the IP address where the resource is located. Otherwise, the local name server queries one of the root name servers, which looks up the second-level domain name and determines which final name server, called the delegated name server, is responsible for resolving the domain name. The local name server then asks the delegated name server for the IP address corresponding to the full domain name you typed in.

For example, if you type in, and your local name server doesn't know the IP address for, it will query the root servers. The root servers will reply that is delegated to a particular name server -- say, the one at -- and your local name server will then ask the delegated name server to resolve the whole domain name,, and the delegated name server will return the IP address to your local name server.

When you register a domain, the registrar will usually specify the IP addresses of at least two delegated name servers. You may want to change the name servers to those of your ISP or web host, or perhaps even operate your own name servers.

Related Links
DNS & BIND (Liu, Cricket, et al. 2001 O'Reilly Press) explains everything you need to know to operate
your own DNS server.