The Class A and B addresses make up 75 percent of the IPv4 address space, however fewer than 17,000 organizations can be assigned a Class A or B network number. Class C network addresses are far more numerous than Class A and Class B addresses, although they account for only 12.5 percent of the possible four billion IP addresses.

Unfortunately, Class C addresses are limited to 254 usable hosts. This does not meet the needs of larger organizations that cannot acquire a Class A or B address. Even if there were more Class A, B, and C addresses, too many network addresses would cause Internet routers to come to a stop under the burden of the enormous size of routing tables required to store the routes to reach each of the networks.

As early as 1992, the Internet Engineering Task Force (IETF) identified the following two specific concerns:

• Exhaustion of the remaining, unassigned IPv4 network addresses. At the time, the Class B space was on the verge of depletion.
• The rapid and large increase in the size of Internet routing tables occurred as more Class C networks came online. The resulting flood of new network information threatened the ability of Internet routers to cope effectively.

Over the past two decades, numerous extensions to IPv4 have been developed. These extensions are specifically designed to improve the efficiency with which the 32-bit address space can be used. Two of the more important of these are subnet masks and classless interdomain routing (CIDR), which are discussed in more detail in later lessons.

Meanwhile, an even more extendible and scalable version of IP, IP Version 6 (IPv6), has been defined and developed. IPv6 uses 128 bits rather than the 32 bits currently used in IPv4. IPv6 uses hexadecimal numbers to represent the 128 bits. IPv6 provides 640 sextrillion addresses. This version of IP should provide enough addresses for future communication needs. Figure shows IPv4 addresses which are 32 bits long, written in decimal form, and separated by periods. IPv6 addresses are 128 bits long, written in hexadecimal form, and separated by colons. IPv6 fields are 16 bits long. To make the addresses easier to read, leading zeros can be omitted from each field. The field :0003: is written :3:. IPv6 shorthand representation of the 128 bits uses eight 16-bit numbers, shown as four hexadecimal digits.

After years of planning and development, IPv6 is slowly being implemented in select networks. Eventually, IPv6 may replace IPv4 as the dominant Internet protocol.

Web Links IPv4 vs. IPv6 http://www.comp.lancs.ac.uk/computing/ users/sschmid/Spie/ node5.html