Background

The World Wide Web (WWW) has become such a normal part of life, with browsing so simplified, that the vast majority take for granted that when they type in an address (aka URL), they load a page.  Why is it, however, that http://www.websitemaven.com leads a user to my site?

Information on the internet is routed according to Internet Protocol Addresses (or IP addresses for short).  Every web server that hosts web pages to be viewed has a unique IP address, such as 192.0.34.35.  You can actually browse to websites using IP addresses but what if you had to remember to type 192.0.34.35 every time you wanted to visit a web page?  This is where the Domain Name System comes in?

In the early days of the Internet, there were few computers connected to the Internet and a central database was maintained that listed every computer on the Internet.  With several thousand computers connected, the database became too large to manage centrally and in 1983, the Domain Name System was invented at USC to distributed databases, decentralizing management while maintaining central control of domains.

The Domain Name System (DNS) uses the familiar address of www.somedomainname.com and translates (or resolves) the name into an IP address.  People naturally remember names or “mnemonic” devices more easily than numbers (which is why we sort telephone books by last name to find phone numbers and not the other way around.)  The goal of the DNS is for any Internet user any place in the world to reach a specific website IP address by entering its domain name.  Domain names are also used for reaching e-mail addresses and for other Internet applications.

In an Internet address – such as icann.org – the .org part is known as a Top Level Domain, or TLD. So-called “TLD registry” organizations house online databases that contain information about the domain names in that TLD. The .org registry database, for example, contains the Internet whereabouts – or IP address – of icann.org. So in trying to find the Internet address of icann.org your computer must first find the .org registry database. How is this done?

How the Domain Name System Works

At the core of the DNS are 13 special computers, called root servers. ICANN coordinates their operation and they are distributed around the world.  All 13 servers contain identical information; this allows the workload to be spread as well as to back each other up.  Reflect on the elegance of the design that allows the central resolution of domain names for millions of people with only 13 servers!

The 13 servers are called root servers and they contain the IP address of all the Top Level Domain (TLD) registrars.  These TLDs include global registries such as .com, .net, .org, etc. and the 244 country specific registries (.tw, .ca, .uk, etc.)

Scattered across the Internet are thousands of computers – called “Domain Name Resolvers” or just plain “resolvers” – that routinely cache the information they receive from queries to the root servers.  These resolvers are located strategically with Internet Service Providers (ISPs) or institutional networks.  They are used to respond to a user’s request to resolve a domain name – that is, to find the corresponding IP address.

When a user types a domain name for a web page or to send an e-mail, a domain name request is forwarded to a local resolver to figure out where to send or look for name.com.  The resolver knows how to find the .com registry as it has copied its IP from a root server beforehand.  The resolver then goes to the .com registry and “says” “Can you give me the IP address for name.com?”  The .com registry returns the IP address of name.com which is sent back to the requesting server so it can make a connection using the IP address provided.  It all happens in fractions of a second (usually) without the user needing to know (nor caring) what the IP address of name.com is.

Top-level, second-level, and third-level domains

The domain name system is a hierarchical system.  You may have encountered domain names like www.usa.xerox.com.  That domain name has different domain levels within it.  The top level domain is .com, controlled by ICANN’s root server.  Moving to the left of .com, xerox.com is a second-level domain – the .com registry contains the IP address of xerox.com.  Moving further left, usa.xerox.com is a third-level domain.  Xerox has its only domain name system (DNS) server that has the IP address of usa.xerox.com.  Xerox’s DNS system also knows where to resolve www.usa.xerox.com where www is at the fourth-level position though is not a domain.  It is rare to see domain names beyond the third level.

Who cares about this domain name stuff?

I thought you did since you clicked on What is a Domain Name and you’re reading this page.  For many, ignorance is bliss as they need not understand the system to use it.  If you are going to purchase or think you want to own a domain name it is useful to understand it.  When you purchase a domain name on the Internet, that domain name is yours – you can point it whereve you want and if you understand how the domain name system work then you have great flexibility as to who you pay to register your domain as well as who you choose to let host your domain.

For instance, I have several websites hosted at Lunar Pages and own several domain names for personal and professional use.  I registered the domain names at the registry of my choice and I can choose where I want to point those domain names to.  I point several to Lunar Pages but if I ever wanted to change Web Hosting companies, I can do so within minutes by pointing the domain names to a different host.  More discussion on managing your domain is available on this site but the main point is that if you understand the fundamental principles of the domain name system (DNS) then you have much more power and flexibility as an Internet user.