Does Android have support for IPv6? I wonder if Android has support for IPv6? I have heard that the phone company I use has support for IPv6 and I have a IPv6 address if I visit http://ipv6.whatismyipv6.net.ipv4.sixxs.org/.

But I can not visit IPv6-only sites like http://ipv6.google.com, why is that? Does Android have support for IPv6?
As of Android 6.0, the majority of the IPv6 protocol seems to be supported with significant parts missing such as DHCPv6.
Check Issue #36908577 Support IPv6 for more info, specifically the issues listed in comment #105:

Enhancement: Support for DHCPv6 (RFC 3315) Won't Fix (Intended behavior)
http://code.google.com/p/android/issues/detail?id=32621
Enhancement: Support for Recursive DNS Server Option in ICMPv6 Router Advertisements (RFC 6106) Fixed
http://code.google.com/p/android/issues/detail?id=32629
Enhancement: Support connecting to IPv6-only wireless networks Fixed
http://code.google.com/p/android/issues/detail?id=32630
Defect: APN Protocol ignored when roaming Fixed
http://code.google.com/p/android/issues/detail?id=32631
Enhancement: Automatic fallback from IPv4/IPv6 APN Protocol to parallel IPv4 + IPv6 Won't Fix (Obsolete)
http://code.google.com/p/android/issues/detail?id=32632
Defect: Frequently ignores ICMPv6 RAs and other multicast traffic Won't Fix (Obsolete)
http://code.google.com/p/android/issues/detail?id=32662

(Consider reading the issues and their comments carefully, because while some of them have their status updated to &quot;fixed&quot;, some users were reporting they still have the same issues.)