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.
The IANA also designated a range of addresses in class A, B and C for use in private networks. These addresses can be used by anyone within their network without any required permission but these addresses are not routable on the Internet. You ISP or your organization usually assigns you one of these addresses and later translates it to a public address when you want to get out to the Internet. The designated ranges for private IP addresses are:
Class A – 10.0.0.0 to 10.255.255.255 (1 network)
Class B – 172.16.0.0 to 172.31.255.255 (16 networks)
Class C – 192.168.0.0 to 192.168.255.255 (256 networks)
Address translation and private IP addresses are discussed in detail in Chapter 9.