The point in time when IPv4 addresses would be completely allocated (and
hence no new networks could attach to the Internet) was the subject of considerable
debate. The estimates of the two leaders of the IETF’s Address Lifetime Expectations
working group were that addresses would become exhausted in 2008 and 2018,
respectively [Solensky 1996]. In February 2011, IANA allocated out the last remaining
pool of unassigned IPv4 addresses to a regional registry. While these registries
still have available IPv4 addresses within their pool, once these addresses are
exhausted, there are no more available address blocks that can be allocated from a
central pool [Huston 2011a]. Although the mid-1990s estimates of IPv4 address
depletion suggested that a considerable amount of time might be left until the IPv4
address space was exhausted, it was realized that considerable time would be needed
to deploy a new technology on such an extensive scale, and so the Next Generation
IP (IPng) effort [Bradner 1996; RFC 1752] was begun. The result of this effort was
the specification of IP version 6 (IPv6) [RFC 2460] which we’ll discuss below. (An
often-asked question is what happened to IPv5? It was initially envisioned that the
ST-2 protocol would become IPv5, but ST-2 was later dropped.) Excellent sources of
information about IPv6 are [Huitema 1998, IPv6 2012].