Behind the scenes, the story becomes a little more complicated.
In an Internet address - such as icann.org - the .org part is known as a Top Level Domain, or TLD. So-called "TLD registry" organisations 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?
At the heart of the DNS are 13 special computers, called root servers. They are coordinated by ICANN and are distributed around the world. All 13 contain the same vital information - this is to spread the workload and back each other up.
Why are these root servers so important? The root servers contain the IP addresses of all the TLD registries - both the global registries such as .com, .org, etc. and the 244 country-specific registries such as .fr (France), .cn (China), etc. This is critical information. If the information is not 100% correct or if it is ambiguous, it might not be possible to locate a key registry on the Internet. In DNS parlance, the information must be unique and authentic. Let us look at how this information is used.
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.
So what happens to a user's request to reach our familiar friend at icann.org? The request is forwarded to a local resolver. The resolver splits the request into its component parts. It knows where to find the .org registry - remember, it had copied that information from a root server beforehand - so it forwards the request over to the .org registry to find the IP address of icann.org. This answer is forwarded back to the user's computer. And we're done. It's that simple! The domain name icann.org has been "resolved"!
Why do we need the resolvers? Why not use the root servers directly? After all, they contain essentially the same information. The answer is for reasons of performance. The root servers could not handle hundreds of billions of requests a day! It would slow users down.
If you are still with the story, you are already wondering about more complicated names with more parts such as www.icann.org. Well, the DNS is a hierarchical system. First, the resolver finds the IP address for the .org registry, queries that registry to find the IP address for icann.org, then queries a local computer at that address to find the final IP address for www.icann.org. Just what you would expect.
It is important to remember the central and critical role played by the root servers that store information about the unique, authoritative root. Confusion would result if there were two TLDs with the same name: which one did the user intend? The beauty of the Internet architecture is that it ensures there is a unique, authoritative root, so that there is no chance of ambiguity.
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