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If we had to remember the IP addresses of all of the Web sites we visit every day, we would all go nuts. Human beings just are not that good at remembering strings of numbers. We are good at remembering words, however, and that is where domain names come in.
You probably have hundreds of domain names stored in your head. For example:
- www.iPaki.com - a typical name
- www.yahoo.com - the world's best-known name
- www.mit.edu - a popular EDU name
- encarta.msn.com - a Web server that does not start with www
- www.bbc.co.uk - a name using four parts rather than three
- ftp.microsoft.com - an FTP server rather than a Web server
The COM, EDU and UK portions of these domain names are called the top-level domain or first-level domain. There are several hundred top-level domain names, including COM, EDU, GOV, MIL, NET, ORG and INT, as well as unique two-letter combinations for every country.
Within every top-level domain there is a huge list of second-level domains. For example, in the COM first-level domain, you've got:
- iPaki
- yahoo
- msn
- microsoft
- plus millions of others...
Every name in the COM top-level domain must be unique, but there can be duplication across domains. For example, howstuffworks.com and howstuffworks.org are completely different machines.
In the case of bbc.co.uk, it is a third-level domain. Up to 127 levels are possible, although more than four is rare. The left-most word, such as www or encarta, is the host name. It specifies the name of a specific machine (with a specific IP address) in a domain.
A given domain can potentially contain millions of host names as long as they are all unique within that domain.
