The database known as the Domain Name System translates IP Numbers into domain names and back.

Domain Name Service; a system that maps user-friendly computer host and domain names (e.g., host.domain.com) to IP addresses (e.g., 204.162.86.58). Like telephone numbers, IP addresses uniquely identify computers on the Internet for addressing messages. See also host name, domain, and IP address.

The Domain Name System is how the Internet links together the thousands of Networks that it is comprised of. The DNS is utilised whenever you send an Email or access a particular Web Page . Each computer on the Internet has a one of more Domain Names such as "fredbloggs.co.uk". The .co indicates a commercial organisation & the .uk indicates that the computer is in the United Kingdom.
Standard conventions used in Domain Names include:-
ac - Educational institution
co - Commercial organisation
com - Commercial organisation
edu - Educational institution
gov - Non military government organisations
int - International Organisations
mil - Military government organisations
net - Networks
org - non profit organisation
You will also see these codes in URL's such as
"homepages.enterprise.net/jenko/Glossary/G.htm".

These DNS converts the Domain Names to a unique number known as an IP address (the IP stands for Internet Protocol). You will often see the IP address displayed by your Web Browser when you are connecting to a particular computer.