As you know already, every host on a network requires a unique IP address. This is easily manageable in a small network but not a network as large as the Internet. The Internet Assigned Numbers Authority (IANA) is responsible for managing and distributing IP addresses. The IANA has created 5 address registrars in five locations of the world. ISPs and large organizations purchase the addresses from these registrars. The end user in turn gets the IP address from the ISP. These purchasable IP addresses are called public addresses and are routable on the Internet. Every host on the Internet has one of these addresses, in theory.