When you sign up with an ISP, part of your setup procedure will almost certainly involve telling your Internet software the IP address of a nameserver on the ISP's network. We'll look at how nameservers detail in the next section. Now I'll explain how the overall system works. Internet hostnames are composed of parts separated by dots. A domain is a collection of machines that share a common name suffix.
Domains can live inside other domains.
Each domain is defined by an authoritative name server that knows the IP addresses of the other machines in the domain. Now here's the important part. In our example, the authoritative name server for the .org domain knows the IP address of the nameserver for .linuxdoc.org, but not the address of all the other machines in linuxdoc.org. The domains in the DNS system are arranged like a big inverted tree. At the top are the root servers.