Given that there are more than 56 million hosts on the Internet, it would be very difficult to keep track of each host. Part of the Internet Protocol requires that each host on the Internet has a unique address. Having a unique address makes it easier for routers and other Internet devices to find a specific host. These numbers are not arbitrarily assigned by the user or network administrator. An organization is assigned a block of IP addresses by the body that governs Internet address use.
Administrators are free to assign addresses only out of the block they ve been given.
Historically, this was the only way that IP addresses could be assigned. Once a number was given to a particular host, that number was used only by that host until that host was taken out of service or the number was reassigned to a different host. Rather than having several blocks of statically assigned IP addresses, some administrators wanted to have the ability to assign IP addresses dynamically. That is, when a host was turned on, the user would have to decide whether he was going to use the Internet or not. If his plans included using the Internet, an IP address would be assigned to that host for the duration of the user s connection.