Dynamic Host Configuration Protocol
To access the Internet, users need an IP address. But there are situations in which there may not be enough IP addresses to serve all users. (This used to happen frequently when networks expanded beyond their original projected size, and administrators discovered that they had not reserved enough address numbers.)
To overcome this shortage, a system to share IP addresses was created, called the Dynamic Host Configuration Protocol. DHCP provided an IP address to those users who were actually logged on at the moment, drawing them from a pool of all available IP addresses. The pool was usually much smaller than the number of users, but that was okay, because all the users were rarely in the office or using the Internet at once. Shortly after the user logged off, the IP address could be reassigned to another user that was just logging on. Oversubscribing, that is, operating with fewer addresses than computers, is a tactic also used by phone companies to provide everybody sharing a common bank of equipment.