Internet Protocol version 6 (IPv6) is the proposed replacement for the widely adopted IPv4. IPv6 is growing in use due to the impending exhaustion of the IPv4 address space.
The most distinguishing feature of IPv6 is the expansion of IPv4's source and destination address fields from 32 bits to 128 bits. This expansion is truly vast. In order of magnitude, the IPv4 space can be measured on the order of four billions. The IPv6 space, by contrast, numbers in the undecillions, a barely imaginable number, which can be factored into a billion, billion, billion, billions.
IPv6 was first proposed in the 1990s, during what was then a projected exhaustion of classful IPv4 addresses. However, due to the radical nature of the switch from IPv4 to IPv6, IPv4's effective life was extended considerably by other technologies including:
A long transition from IPv4 to IPv6 is currently projected, due to the huge installed base of IPv4 equipment and software. Most fairly recent systems are IPv6 capable. The most likely transition path from IPv4 to IPv6 includes various coexistence approaches such as:
See RFC 2640 for the original IPv6 specification.