Today almost everyone who work in internet have heard about top level domains (often abbreviated as TLD).
What not everyone knows, even between those who work in internet, is that there is a domain above the top level domains. It’s the . domain.
This is the domain under which (almost) all other domains are. And I say almost because there are tricks related with the way DNS works to create domains (more or less) private that are not possible to resolve using what I will call the traditional name resolution cascade.
Names resolved by DNS are something we got used to see everyday. And that includes even people who don’t even use internet. They are everywhere - in the cars of the companies, in the invoices, in the business cards, professional and personal - they are the address of the blogs, of the social websites and even part of the email addresses. All of this names are resolved using DNS.
But how are they structured?
Well, first we have the lord of all the domains, the domain . – which is not usually used explicitly – and under that we have all the top level domains – com, net, org, pt, br, uk, us, nl, … – there used to be few global domains – not there are a lot of them –, every country have one and even some regions have one – like eu or asia.
Each top level domain have an entity that manages the domains that are created under that TLD. For instance, for the TLD .pt the entity that manages that domain is the FCCN (fccn.pt).
Usually, provided that some conditions are met, it is possible to create domains under this top level domains, or under a subdomains of them - for instance, the uk have the subdomain .co.uk under which domains can be registered - and the management of this domains is given to the entity that registers it, who can then create more subdomains under the domains they registered.
Each of this names/domains is separated from (or joined to) the domain above them by a ., generating this way the names that we are already used to see as the names of the websites.
Traditionally, the first name represent the name of the server, and all the other names are - in a way - the path we need to walk to find that server.
As an example, when we have the address www.neversmind.site, we are looking at the server named www of the domain neversmind that is under the TLD site.
Today, however, with faster servers and a much bigger diversity of services, a server often serves multiple services and multiples sites, and the first part is more often the name of the service than the name of the server where the service is provided from.
Today we have as often multiple services being served from a single server as we have multiple servers serving a single services, as well as multiple servers serving multiple services together.