At this time, most websites use only IPv4. It is the 4th revision in the development of the Internet Protocol (IP) and it is the first version of the protocol to be widely deployed. IPv4 uses 32-bits for an IP address and thus allows for a theoretical maximum of 232 IP Numbers (approximately four billion). This is no doubt a huge figure. But, the explosion in the number of people, devices, and web services on the Internet means that IPv4 is running out of space.
IPv6 is the successor to IPv4 which uses 128-bit addresses, and thus allows a maximum of 2128 IP Numbers. It means that every device on the planet will have its own IP Number. Implementation of this new protocol has totally faded up the scarcity of IP allotment.
On June 8, 2011 - World IPv6 Day - major web companies, including Google, Facebook, Yahoo!, and Bing, tested IPv6 on their main websites for 24 hours. But by 6 June 2012, all major ISPs, home networking equipment manufacturers, and web companies around the world are coming together to permanently enable IPv6 for their products and services. IPv6 will be enabled in addition to IPv4, so if you don't have IPv6, don't worry - you will simply use IPv4 as usual. But you can test for it, by this testing tool from Google.
How to Test
Point your browser to ipv6test.google.com, and it will show you the result, whether your computer and connection are ready or not for IPv6. This is the result for mine.
For more advance results and technical details use test-ipv6.com.