The Domain Name System (DNS) maps names to IP addresses for the same reason port names map to numbers, people are better with names than numbers.
For example, it is easier for a user to remember www.mit.edu than to remember 126.96.36.199. With DNS, the computer does not require the IP number because it can always look up any name you give it.
To name hosts, DNS uses a tree-oriented classification scheme, and a notation similar to the dotted quad syntax.
For example, your friend's email address is at:
Reading left to right, the name before the first dot is the host name, the name after the last dot is the top-level domain, and all intervening names are networks in between the host and the top-level domain.
In example above, the DNS name "charger.mit.edu" indicates a host named "charger" in the "mit" network and the "edu" top-level domain.
The following MouseOver shows the parts of a DNS tree:
DNS Viewed as Tree
Whenever a DNS name has a host name, a network name, and a top-level domain (as in "charger.mit.edu"), the name is "fully-qualifed."
Fully-qualified domain names (FQDN) describe a single host on the Internet.
Examples of other fully-qualified domain names include: