Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 85
Prefix Assignments

We must look at this concept through the ISP networks. ISPs give us the ability to communicate over the Internet. You cannot simply attach to the Internet unless you connect with an ISP. ISPs come is a variety of flavors: some are large and provide access to other ISPs and individuals, and some are small and only provide Internet connectivity to individuals and businesses. ISPs are allocated blocks of addresses that are contiguous in range. The concept first used Class C addresses since Class B addresses were exhausted and Class A addresses were not handed out (they are being handed out today). The basic idea of the plan is to allocate blocks of Class C (first, other Class A and B addresses to follow) network numbers to each network service provider. (It is very helpful here to read RFC 2050 before continuing this section). The customers of these providers are then allocated bit mask-oriented subnets of the service provider’s address. The assignment blocks to the IR can be found at the end of this section.