IPv6 is designed to be a simple, forward-compatible upgrade to the existing version of IP. This upgrade is also intended to resolve all the weaknesses that IPv4 is currently manifesting including: the shortage of available IP addresses, the inability to accommodate time-sensitive traffic, and its lack of network layer security.
IPv6 is sometimes referred to as IPng, or IP: Next Generation. IPng was the name of the original initiative to develop a next generation IP. As the protocol's specifications were being developed, the protocol was named IP version 6 (which was then abbreviated to IPv6).
Although IPng and IPv6 really refer to different things (that is, a development initiative versus a protocol), ...