You've seen lots of domains: microsoft.com, harvard.edu, yahoo.co.uk and, of course, cream.org. Of course, the phrase cream.org does very little on its own, but type ice.cream.org into a browser and, voilla, a web page presents itself. A name like ice.cream.org, which consists of the hostname ice prefixed to the domain name cream.org, so that it actually points to a specific machine, is called a Fully Qualified Domain Name. In fact, the FQDN resolves to an IP address (see the TCP/IP section if you don't know what an IP address is). If this is confusing you, think of the domain name as the name of your street and city. Think of the hostname as your house's name (or number). Think of the fully qualified domain name as both together.
The DNS (domain name system) is the world-wide distributed database that runs over the Internet across thousands of domain name servers. The system converts, in the background, the requested domain names (which we like to type) into the IP address (which the computers can actually deal with).
Each domain (cream.org, microsoft.com and so forth) has an authoratitive server somewhere in the world which keeps the master record for this particular domain. Ice happens to run the domain-name server that is authoratitive for its own cream.org domain (as well as others). If the domain name server on ice.cream.org fails and is uncontactable, specified secondary "slave" servers maintain recent copies of the authoratitive records. In this way, it should usually be possible to determine which domain name points to which host, even when things on the host's main nameserver are going wrong.
Each authoratitive name server can decide what particular host-names are tacked onto the general domain name. So at cream.org, ice.cream.org, double.cream.org and whipped.cream.org are three fully qualified domain names on the cream.org domain name. They could point to separate machines, but in our case, they point to different web pages on the same machine. It is in our gift to set up anything.cream.org, and that is the beauty of the domain name system - you do not have to have one central server in the "middle of the Internet" determining everyone's nomenclature. At the moment, any queries to do with the cream.org domain are referred to the domain name server on ice.cream.org. Any domain names requested for microsoft.com are dealt with by a machine in the bowels of Redmond - dns1.microsoft.com, which has the IP address of 131.107.1.240
As with IP address, there is a central registry which deals with the registration of domain names. In America, this organisation is called INTERNIC. There are several common "top-level" domain names, like ".com", ".edu" and so forth, with more on the horizon. Some countries use Geographical monickers, like "co.uk" (which means "a commercial service in the United Kingdom") or "ac.za" (which signifies an "academic service in South Africa"). The ".org" in cream.org implies we are a non-geographical non-profit organisation of some sort. Which I guess we are.
The domain name system contains other records too - for example, information about where mail for a particular domain should be delivered and so forth. These are known as MX (Mail eXchange) records.
For the moment, your IP number is: 3.142.211.95
Your current fully-qualified domain name is: (none)
Our web-server knew your IP number and could look up your host and domain name using the name server on this machine.