Due to a theoretical shortage of 4.3 billion unique public IP addresses in the familiar four-octet format (ie: 192.168.0.100), IPv6 uses hexadecimal to draw from a pool that is 3.4 X 10^38, or 340 followed by thirty six zeroes unique addresses. For context, this is 56 octillion addresses for each person on the planet.
However, due to things like classful network design and the typical segregation of public & private LANs to just a few crossover points, the finite limitation of IPv4 is rarely mentioned. If you're buying network gear, there is always a big emphasis on 'future proofing' your designs with IPv6 equipment, but I've never seen it actually used.
Do you use IPv6 addresses? Please vote: