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The Basics

When you use the Web or send an e-mail message, you use a domain name to do it. For example, the URL "http://www.gazoobie.com" contains the domain name gazoobie.com. So does the e-mail address "iknow@gazoobie.com."

Human-readable names like "gazoobie.com" are easy for people to remember, but they don't do machines any good. All of the machines use names called IP addresses to refer to one another. For example, the machine that humans refer to as "www.gazoobie.com" has the IP address 216.145.103.120. Every time you use a domain name, you use the Internet's domain name servers (DNS) to translate the human-readable domain name into the machine-readable IP address. During a day of browsing and e-mailing, you might access the domain name servers hundreds of times!

Domain name servers translate domain names to IP addresses. That sounds like a simple task, and it would be -- except for five things:

  • There are billions of IP addresses currently in use, and most machines have a human-readable name as well.
  • There are many billions of DNS requests made every day. A single person can easily make a hundred or more DNS requests a day, and there are hundreds of millions of people and machines using the Internet daily.
  • Domain names and IP addresses change daily.
  • New domain names get created daily.
  • Millions of people do the work to change and add domain names and IP addresses every day.
The DNS system is a database, and no other database on the planet gets this many requests. No other database on the planet has millions of people changing it every day, either. That is what makes the DNS system so unique!

IP Addresses

To keep all of the machines on the Internet straight, each machine is assigned a unique address called an IP address. IP stands for Internet protocol, and these addresses are32-bit numbers normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this: 

          216.183.103.150

The four numbers in an IP address are called octets because they can have values between 0 and 256 (28 possibilities per octet).

Every machine on the Internet has its own IP address. A server has a static IP address that does not change very often. A home machine that is dialing up through a modem often has an IP address that is assigned by the ISP when you dial in. That IP address is unique for your session and may be different the next time you dial in. In this way, an ISP only needs one IP address for each modem it supports, rather than for every customer.

If you are working on a Windows machine, you can view your current IP address with the command WINIPCFG.EXE (IPCONFIG.EXE for Windows 2000/XP). On a UNIX machine, type nslookup along with a machine name (such as "nslookup www.howstuffworks.com") to display the IP address of the machine (use the command hostname to learn the name of your machine).

For more information on IP addresses, see www.IANA.org.

As far as the Internet's machines are concerned, an IP address is all that you need to talk to a server. For example, you can type in your browser the URL http://216.183.103.150 and you will arrive at the machine that contains the Web server for gazoobie. Domain names are strictly a human convenience.

Domain Names

If we had to remember the IP addresses of all of the Web sites we visit every day, we would all go nuts. Human beings just are not that good at remembering strings of numbers. We are good at remembering words, however, and that is where domain names come in. You probably have hundreds of domain names stored in your head. For example:
  • www.gazoobie.com - a typical name
  • www.yahoo.com - the world's best-known name
  • www.mit.edu - a popular EDU name
  • encarta.msn.com - a Web server that does not start with www
  • www.bbc.co.uk - a name using four parts rather than three
  • ftp.microsoft.com - an FTP server rather than a Web server
The COM, EDU and UK portions of these domain names are called the top-level domain or first-level domain. There are several hundred top-level domain names, including COM, EDU, GOV, MIL, NET, ORG and INT, as well as unique

Distributing Domain Names

Because all of the names in a given domain need to be unique, there has to be a single entity that controls the list and makes sure no duplicates arise. For example, the COM domain cannot contain any duplicate names, and a company called Network Solutions is in charge of maintaining this list. When you register a domain name, it goes through one of several dozen registrars who work with Network Solutions to add names to the list. Network Solutions, in turn, keeps a central database known as the whois database that contains information about the owner and name servers for each domain. If you go to the whois form, you can find information about any domain currently in existence.
While it is important to have a central authority keeping track of the database of names in the COM (and other) top-level domain, you would not want to centralize the database of all of the information in the COM domain. For example, Microsoft has hundreds of thousands of IP addresses and host names. Microsoft wants to maintain its own domain name server for the microsoft.com domain. Similarly, Great Britain probably wants to administrate the uk top-level domain, and Australia probably wants to administrate the au domain, and so on. For this reason, the DNS system is a distributed database. Microsoft is completely responsible for dealing with the name server for microsoft.com -- it maintains the machines that implement its part of the DNS system, and Microsoft can change the database for its domain whenever it wants to because it owns its domain name servers.
Every domain has a domain name server somewhere that handles its requests, and there is a person maintaining the records in that DNS. This is one of the most amazing parts of the DNS system -- it is completely distributed throughout the world on millions of machines administered by millions of people, yet it behaves like a single, integrated database!

Creating a New Domain Name

When someone wants to create a new domain, he or she has to do two things:
  • Find a name server for the domain name to live on.
  • Register the domain name.
Technically, there does not need to be a machine in the domain -- there just needs to be a name server that can handle the requests for the domain name.

There are two ways to get a name server for a domain:

  • You can create and administer it yourself.
  • You can pay an ISP or hosting company to handle it for you.
Most larger companies have their own domain name servers. Most smaller companies pay someone.

The history of gazoobie.com is typical. When gazoobie was first created, it began as a parked domain. This domain lived with our company called www.gazoobie.com. We maintained the name server and also maintained a machine that created the single "under construction" page for the domain.

To create a domain, you fill out a form with a company that does domain name registration (examples: register.com, verio.com, networksolutions.com). They create an "under construction page," create an entry in their name server, and submit the form's data into the whois database. Twice a day, the COM, ORG, NET, etc. name servers get updates with the newest IP address information. At that point, a domain exists and people can go see the "under construction" page.

We then started publishing content under the domain www.gazoobie.com. We set up a hosting account with Tabnet (now part of Verio, Inc.), and Tabnet ran the DNS for gazoobie as well as the machine that hosted the gazoobie.com Web pages. This type of machine is called a virtual Web hosting machine and is capable of hosting multiple domains simultaneously. Five-hundred or so different domains all shared the same processor.

As gazoobie.com became more popular, it outgrew the virtual hosting machine and needed its own server. At that point, we started maintaining our own machines dedicated to HowStuffWorks, and began administering our own DNS. We have a primary server and a secondary:

  • AUTH-NS1.gazoobie.COM  
  • AUTH-NS2.gazoobie.COM  
Our primary DNS is gazoobie.com. Any changes we make to it propagate automatically to the secondary, which is also maintained by our ISP.

All of these machines run name server software called BIND. BIND knows about all of the machines in our domain through a text file on the main server that looks like this:

            @         NS    auth-ns1.gazoobie.com.
        @         NS    auth-ns2.gazoobie.com.
        @         MX 10 mail

        mail      A     209.170.137.42

        vip1      A     216.183.103.150
        www       CNAME vip1
Decoding this file from the top, you can see that:
  • The first two lines point to the primary and secondary name servers.

  • The next line is called the MX record. When you send e-mail to anyone at gazoobie.com, the piece of software sending the e-mail contacts the name server to get the MX record so it knows where the SMTP server for gazoobie is. Many larger systems have multiple machines handling incoming e-mail, and therefore multiple MX records.

  • The next line points to the machine that will handle a request to mail.gazoobie.com.

  • The next line points to the IP address that will handle a request to oak.gazoobie.com.

  • The next line points to the IP address that will handle a request to gazoobie.com (no host name).

You can see from this file that there are several physical machines at separate IP addresses that make up the gazoobie server infrastructure. There are aliases for hosts like mail and www. There can be aliases for anything. For example, there could be an entry in this file for scoobydoo.gazoobie.com, and it could point to the physical machine called walnut. There could be an alias for yahoo.gazoobie.com, and it could point to yahoo. There really is no limit to it. We could also create multiple name servers and segment our domain.

The Beauty of DNS
As you can see from this description, DNS is a rather amazing distributed database. It handles billions of requests for billions of names every day through a network of millions of name servers administered by millions of people. Every time you send an e-mail message or view a URL, you are making requests to multiple name servers scattered all over the globe. What's amazing is that the process is usually completely invisible and extremely reliable!



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