Domain Name 

The unique name that identifies an Internet site. Domain Names always have 2 or more parts, separated by dots. The part on the left is the most specific, and the part on the right is the most general. A given machine may have more than one Domain Name but a given Domain Name points to only one machine. For example, the domain names: mirchmasala.net, ftp.mirchmasala.net, whatever.mirchmasala.net can all refer to the same machine, but each domain name can refer to no more than one machine. Usually, all of the machines on a given Network will have the same thing as the right-hand portion of their Domain Names in the examples above. It is also possible for a Domain Name to exist but not be connected to an actual machine. This is often done so that a group or business can have an Internet e-mail address without having to establish a real Internet site. In these cases, some real Internet machine must handle the mail on behalf of the listed Domain Name.

DNS

The Domain Name System (abbreviated DNS) is an Internet directory service. DNS is how domain names are translated into IP addresses, and DNS also controls email delivery. If your computer cannot access DNS, your web browser will not be able to find web sites, and you will not be able to receive or send email. 

The DNS system consists of three components: DNS data (called resource records), servers (called name servers), and Internet protocols for fetching data from the servers. The billions of resource records in the DNS are split into millions of files called zones. Zones are kept on authoritative servers distributed all over the Internet, which answer queries based on the resource records stored in the zones they have copies of. 
Caching servers ask other servers for information and cache any replies. Most name servers are authoritative for some zones and perform a caching function for all other DNS information. Large name servers are often authoritative for tens of thousands of zones, but most name servers are authoritative for just a few zones. 

(1) Short for Domain Name System (or Service or Server), an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.example.com might translate to 
123.123.123.1. The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned. 

(2) Short for digital nervous system, a term coined by Bill Gates to describe a network of personal computers that make it easier to obtain and understand information.

How does a DNS server work?

The domain name system is a global network of servers that translate host names like www.mirchmasala.com into numerical IP (Internet Protocol) addresses, like 204.62.131.129, which computers on the Net use to communicate with each other. Without DNS, we'd all be memorizing long numbers instead of intuitive URLs or email addresses. And that wouldn't be much fun, would it? 

First, a little history 

Paul Mockapetris designed DNS in 1984 to solve escalating problems with the old name-to-address mapping system. The old system consisted of a single file, known as the host table, maintained by the Stanford Research Institute's Network Information Center (SRI-NIC). As new host names trickled in, SRI-NIC would add them to the table - a couple times a week. Systems administrators would grab the newest version (via FTP) and update their domain name servers. 

But as the Net grew, the host table became unwieldy. Though it worked fine for name-to-address mapping, it wasn't the most practical or effective way to update and distribute the information. And since the stability of the rapidly growing Internet was at stake, Mockapetris and some other folks decided to find a better way. 

Enter DNS. The great thing about the domain name system is that no single organization is responsible for updating it. It's what's known as a distributed database; it exists on many different name servers around the world, with no one server storing all the information. Because of this, DNS allows for almost unlimited growth. 

The Domain Name Space 

In order to understand how a DNS server works, you should be familiar with what is called the domain name space. It sounds a little ominous, I know, but really it's quite simple. In fact, you've probably seen it at one time or another represented by an inverted tree that looks something like this: 

Each node on the tree represents a domain. Everything below a node falls into its domain. One domain can be part of another domain. For example, the machine chichi is part of the .us domain as well as the .com domain. You'll see why this is important in just a minute.
A DNS server is just a computer that's running DNS software. Since most servers are Unix machines, the most popular program is BIND (Berkeley Internet Name Domain), but you can find software for the Mac and the PC as well. DNS software is generally made up of two elements: the actual name server, and something called a resolver. The name server responds to browser requests by supplying name-to-address conversions. When it doesn't know the answer, the resolver will ask another name server for the information. 

To see how it works, let's go back to the domain-name-space inverted tree. 

When you type in a URL, your browser sends a request to the closest name server. If that server has ever fielded a request for the same host name (within a time period set by the administrator to prevent passing old information), it will locate the information in its cache and reply. 

If the name server is unfamiliar with the domain name, the resolver will attempt to "solve" the problem by asking a server farther up the tree. If that doesn't work, the second server will ask yet another - until it finds one that knows. (When a server can supply an answer without asking another, it's known as an authoritative server)

Once the information is located, it's passed back to your browser, and you're sent on your merry way. Usually this process occurs quickly, but occasionally it can take an excruciatingly long time (like 15 seconds). In the worst cases, you'll get a dialog box that says the domain name doesn't exist - even though you know damn well it does. 

This happens because the authoritative server is slow replying to the first, and your computer gets tired of waiting so it times-out (drops the connection). But if you try again, there's a good chance it will work, because the authoritative server has had enough time to reply, and your name server has stored the information in its cache.

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