Is IPv6 multicast routable over the Internet?
So will I be able to access a multicast group over the internet or will it only be supported through private networks like in ipv4?
I must correct a assumption you seem to have made here.
If all the routers between you and your destination support it, then Multicast can certainly work over the IPv4 Internet. It is simply blocked or not configured in many places. I suspect this is because multicast is not well understood, and many people believe they do not need it. So they simply do not permit it through their firewalls/routers.
IPv6 certainly is capable just like IPv4 of having Multicast work globally. Only time will tell us if people actually permit multicast through their networks.
IPv6 public -unicast- addresses have the prefix 2000::/3 (so far). The allocation for multicast includes accommodation for link-local, variously local scoped and global addressing (as per RFC3307). It's the same basic idea as IPv4 multicast, where a chunk of the 224/4 space is set aside for GLOP addresses and such.
Check out RFC3306 as well, I think it probably more directly answers your question.
This specification defines an extension to the multicast addressing architecture of the IP Version 6 protocol. The extension presented in this document allows for unicast-prefix-based allocation of multicast addresses. By delegating multicast addresses at the same time as unicast prefixes, network operators will be able to identify their multicast addresses without needing to run an inter-domain allocation protocol.
So the idea is that if you have a globally routed /64 that it can be included in the overall group ID to give you something that can be routed globally. In other words, if you already have a v6 prefix and a suitably enabled carrier then you're already set.
The standards allow global multicast routing, but AFAIK currently most ISPs restrict multicast only for their use (IPTV etc.)
My own guess is that many ISP's that happen to be cable providers who deploy IPv6 will block it at their border.
It may be cynical of me to think this, but they have a vested interest in keeping the costs HIGH for their content competitors. v6 multicast would allow HBO or any other streaming video provider to transmit one v6 stream to the Ipv6 world over a multicast channel and dramatically cut costs.