Classful IP Addressing

When the ARPANET was commissioned in 1969, no one anticipated that the Internet would explode out of the humble beginnings of this research project. By 1989, ARPANET had been transformed into what we now call the Internet. Over the next decade, the number of hosts on the Internet grew exponentially, from 159,000 in October 1989, to over 72 million by the end of the millennium. As of January 2007, there were over 433 million hosts on the Internet.Without the introduction of VLSM and CIDR notation in 1993 (RFC 1519), Name Address Translation (NAT) in 1994 (RFC 1631), and private addressing in 1996 (RFC 1918), the IPv4 32-bit address space would now be exhausted.
Links: "ISC Domain Survey: Number of Internet Hosts,"

The High Order Bits
IPv4 addresses were initially allocated based on class. In the original specification of IPv4 (RFC 791) released in 1981, the authors established the classes to provide three different sizes of networks for large, medium and small organizations. As a result, class A, B and C addresses were defined with a specific format for the high order bits. High order bits are the left-most bits in a 32-bit address.
Class A addresses begin with a 0 bit. Therefore, all addresses from to belong to class A. The address is reserved for default routing and the address is reserved for loopback testing.
Class B addresses begin with a 1 bit and a 0 bit. Therefore, all addresses from to belong to class B.
Class C addresses begin with two 1 bits and a 0 bit. Class C addresses range from to
The remaining addresses were reserved for multicasting and future uses. Multicast addresses begin with three 1s and a 0 bit. Multicast addresses are used to identify a group of hosts that are part of a multicast group. This helps reduce the amount of packet processing that is done by hosts, particularly on broadcast media. In this course, you will see that the routing protocols RIPv2, EIGRP, and OSPF use designated multicast addresses.
IP addresses that begin with four 1 bits were reserved for future use.
"Internet Protocol,"
"Internet Multicast Addresses,"

The IPv4 Classful Addressing Structure :
The designations of network bits and host bits were established in RFC 790 (released with RFC 791). As shown in the figure, class A networks used the first octet for network assignment, which translated to a classful subnet mask. Because only 7 bits were left in the first octet (remember, the first bit is always 0), this made 2 to the 7th power or 128 networks.
With 24 bits in the host portion, each class A address had the potential for over 16 million individual host addresses. Before CIDR and VLSM, organizations were assigned an entire classful network address. What was one organization going to do with 16 million addresses? Now you can understand the tremendous waste of address space that occurred in the beginning days of the Internet, when companies received class A addresses. Some companies and governmental organizations still have class A addresses. For example, General Electric owns, Apple Computer owns, and the U.S. Postal Service owns (See the link "Internet Protocol v4 Address Space" below for a listing of all the IANA assignments.)
Class B was not much better. RFC 790 specified the first two octets as network. With the first two bits already established as 1 and 0, 14 bits remained in the first two octets for assigning networks, which resulted in 16,384 class B network addresses. Because each class B network address contained 16 bits in the host portion, it controlled 65,534 addresses. (Remember, 2 addresses were reserved for the network and broadcast addresses.) Only the largest organizations and governments could ever hope to use all 65,000 addresses. Like class A, class B address space was wasted.
To make things worse, class C addresses were often too small! RFC 790 specified the first three octets as network. With the first three bits established as 1 and 1 and 0, 21 bits remained for assigning networks for over 2 million class C networks. But, each class C network only had 8 bits in the host portion, or 254 possible host addresses.

"A Brief History of the Internet,"
"Internet Protocol v4 Address Space,"


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