In this chapter, you have seen that IPv4 addressing faces two major
problems: the depletion of addresses, particularly the key
medium-sized space (Class B), and dangerous overgrowth of Internet
routing tables.
In the early 1990s, CIDR ingeniously built on the concept of the
address mask and stepped forward to temporarily alleviate these
crushing problems. The hierarchical nature of CIDR dramatically
improved IPv4's scalability. Once again, a hierarchical design
proves to be a scalable one.
Yet even with subnetting (1985), variable-length subnetting (1987),
and CIDR (1993), a hierarchical structure could not save IPv4 from
one simple problem: There just are not enough addresses to meet
future need. At roughly 4 billion possibilities, the IPv4 address
space is formidable, but it will not suffice in a future world of
mobile Internet-enabled devices and IP-addressable household
appliances (RFC 2235 references the world's first "Internet
toaster").
Recent short-term IPv4 solutions to the address crunch, such as RFC
1918, which sets aside addresses for unlimited internal use, and
NAT, which allows thousands of hosts to access the Internet with
only a handful of valid addresses.
However, the ultimate solution to the address shortage is the
introduction of IPv6 and its 128-bit address. Developed to create a
supply of addresses that would outlive demand, IPv6 is on course to
eventually replace IPv4. The fantastically large address space of
IPv6 will provide not only far more addresses than IPv4, but
additional levels of hierarchy as well. For the record, 128 bits
allows for 340,282,366,920,938,463,
463,374,607,431,768,211,456
possibilities.
In 1994, the IETF proposed IPv6 in RFC 1752, and a number of working
groups were formed in response. IPv6 tackles issues such as address
depletion, quality of service, address autoconfiguration,
authentication, and security.
It will not be easy for organizations deeply invested in the IPv4
scheme to migrate to a totally new architecture. As long as IPv4 (with its recent extensions and CIDR-enabled
hierarchy) remains
viable, administrators will shy away from adopting IPv6. A new IP
protocol requires new software, new hardware, and new methods of
administration. It is likely that IPv4 and IPv6 will coexist, even
within an autonomous system, for years to come.
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