Internet services and apllications usually do not have to use the numeric IP-addresses.
Mostly there are logical names like email@example.com or www.webmuseum.org.
In former times of the ARPANET, the resolving of an IP-address to its logical address was done by a seperate file called hosts.txt which had to be stored on evey single computer.
With the rapid growth of the web, this method became unmanageable.
Out of this problem the idea of a domain name system was born.
This system is build on an logical structure of name server which distribute the information necessary to resolve an logical name to its corresponding numeric value.
You can use the tool on the right hand side to look up some IP-addresses for a logical name.