Internet Protocol version 6 (IPv6) is the next-generation Internet Layer protocol for packet-switched internetworks and the Internet. IPv4 is the dominant Internet Protocol version, and was the first to receive widespread use. In December 1998, the Internet Engineering Task Force (IETF) designated IPv6 as the successor to version 4 by the publication of a Standards Track specification, RFC 2460.

In December 2008, despite celebrating its 10th anniversary as a Standards Track protocol, IPv6 was only in its infancy in terms of general worldwide deployment. A 2008 study by Google indicated that penetration was still less than one percent of Internet-enabled machines in any country. The leaders were Russia (0.76%), France (0.65%), Ukraine (0.64%), Norway (0.49%), and the United States (0.45%). Although Asia lead in terms of absolute deployment numbers, the relative penetration was smaller (e.g., China: 0.24%). IPv6 has been implemented on all major operating systems in use in commercial, business, and home consumer environments. According to the study, Mac OS X lead in IPv6 penetration of 2.44%, followed by Linux (0.93%) and Windows Vista (0.32%).

IPv6 has a much larger address space than IPv4. This results from the use of a 128-bit address, whereas IPv4 uses only 32 bits. The new address space thus supports 2¹²⁸ (about 3.4×10³⁸) addresses. This expansion provides flexibility in allocating addresses and routing traffic and eliminates the primary need for network address translation (NAT). NAT gained widespread deployment as an effort to alleviate IPv4 address exhaustion.

IPv6 also implements new features that simplify aspects of address assignment (stateless address autoconfiguration) and network renumbering (prefix and router announcements) when changing Internet connectivity providers. The IPv6 subnet size has been standardized by fixing the size of the host identifier portion of an address to 64 bits to facilitate an automatic mechanism for forming the host identifier from Link Layer media addressing information (MAC address).

Network security is integrated into the design of the IPv6 architecture. Internet Protocol Security (IPsec) was originally developed for IPv6, but found widespread optional deployment first in IPv4 (into which it was back-engineered). The IPv6 specifications mandate IPsec implementation as a fundamental interoperability requirement.

The general requirements for implementing IPv6 on a network host are specified in RFC 4294.

Motivation for IPv6

The first publicly used version of the Internet Protocol, Version 4 (IPv4), provides an addressing capability of about 4 billion addresses (2³²). This was deemed sufficient in the early design stages of the Internet when the explosive growth and worldwide penetration of networks was not anticipated.

During the first decade of operation of the TCP/IP-based Internet, by the late 1980s, it became apparent that methods had to be developed to conserve address space. In the early 1990s, even after the introduction of classless network redesign, it became clear that this would not suffice to prevent IPv4 address exhaustion and that further changes to the Internet infrastructure were needed.RFC 1752: The Recommendation for the IP Next Generation Protocol By the beginning of 1992, several proposed systems were being circulated, and by the end of 1992, the IETF announced a call for white papers (RFC 1550) and the creation of the "IP Next Generation" (IPng) area of working groups.