DNS:

On the Internet, computers have an IP address which
is a purely numerical address (e.g. 216.71.173.244).
These addresses are not easy for people to remember and
are not descriptive of the website. The Domain Name
System was set up which converts meaningful domain names
into numerical addresses. As the number of websites
increases the current 12 digit IP addresses are running
out. ISPs have a pool of IP addresses which are
dynamically allocated to users each time the connect
and then returned to the pool when then disconnect.