In basic terms IPv6 takes over and/or adds from IPv4 which has 4 billion web addresses and the core of the internet. As the internet has expanded over the years the IP addresses for websites links (as in your http://www.xxxx.com) have started to run out so a new protocol was needed, and here comes in IPv6.
“On 8 June, 2011, Google, Facebook, Yahoo!, Akamai and Limelight Networks will be amongst some of the major organisations that will offer their content over IPv6 for a 24-hour “test drive”. The goal of the Test Drive Day is to motivate organizations across the industry – Internet service providers, hardware makers, operating system vendors and web companies – to prepare their services for IPv6 to ensure a successful transition as IPv4 addresses run out.”
You can test your IPv6 readiness HERE
If you are not IPv6 ready then some sites when they go live with IPv6, you may not be able to view unless they stack and have dual IPv4 and IPv6 versions of their sites.
Many users of Windows have due to some network connection issues, have disabled IPv6, so come June 8th 2011 it maybe a wise thing to re-enable IPv6 and its found in your Network Connection Properties.