To be certain that the IPv6 protocol is being used to access a website, substitute an IPv6 literal address surrounded by square brackets in the address bar in place of a domain name. For example:
http://ipv6.test-ipv6.com
becomes
http://[2001:470:1:18::115].
For additional information, refer to Request For Comments (RFC) 3986 “Uniform Resource Identifier (URI): Generic Syntax".
(Caution: This substitution can sometimes fail. Explanation of possible reasons for this are available. If you encounter problems, review the Broken User FAQ article found on that website for several possible explanations. For even more possible explanations, review this article on the ARIN IPv6 wiki.)
Most non-Apple browsers and IPv6 enabled applications attempt to use IPv6 before IPv4 if the local system and the remote server have v6 addresses. Here’s why debugging IPv6 web connectivity with Safari was historically difficult. It was only with the release of Safari 2.0 with MacOS 10.4 Tiger circa 2005 that Safari gained full IPv6 support; until that release it tried IPv6 first and then fell back to IPv4 if IPv6 didn’t work.