2.9 IPv6
2.9.1 IP address issues and solutions
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.