I see multiple answers assuming you have mistaken what multicast is. You have made no mistake and your question is clear. I asked this question to myself:
Can I multicast across the IPv6 Internet?
Traditionally, such as in IPv4 I would need to request a permanent global multicast address (or subnet) and assign these to my network. This is still possible with IPv6. However, IPv6 is inherently multicast-happy so it would make sense to include some mechanism for me to multicast to you without requesting unique addresses.
The advantages become obvious when you have three people each on two physically separate networks (six people) playing the same network game. The options are to unicast packets to every player (five packets sent for every update), or to multicast (one or two packets for every update): the first packet would be sent to a link-local multicast address for the players on the local LAN, and another packet would be sent to the global multicast address, which the routers would understand are intended for the players on the other LAN. It may even be the case that the packet is sent once to the global multicast address, and the router (or the local clients) know how to deal with that. The latter would certainly be more efficient.
Given how useful multicast is, it would irritate IANA if they had to assign multicast prefixes for everyone who wants to play a network game, or deliver a video conference, or broadcast a live performance to friends, and so on and so on....
The application form from IANA clearly states that you probably don't need to ask for a permanent IPv6 multicast address, which is nice.
Unicast-Prefix-based IPv6 Multicast Addresses
This, of course, has been addressed. The title "Unicast-Prefix-based IPv6 Multicast Addresses" should really say it all: if you have a globally unique IPv6 IP address then you (your computer/device) can assign your(/it)self a globally unique multicast address which is based on your unicast assignment. The requirements are that the software at every point (servers, routers, clients) knows what it's doing. Old routers and lazy ISPs are likely to be the downfall for the next few years.
It was incredibly difficult to find an answer to what seems to be a very simple question, and the closest I could come to find a definitive answer was in RFC3306:
The following are a few examples of the structure of unicast prefix-
based multicast addresses.
- Global prefixes - A network with a unicast prefix of
3FFE:FFFF:1::/48 would also have a unicast prefix-based
multicast prefix of FF3x:0030:3FFE:FFFF:0001::/96 (where 'x'
is any valid scope).
- SSM - All IPv6 SSM multicast addresses will have the format
Most articles (and answers) on IPv6 multicast focus on local multicast which has pre-defined addresses, and are not very helpful. The clincher is that a client can assign itself a unique multicast address based on its unicast address, and of course scopes still apply:
The scope of the unicast-prefix based multicast address MUST NOT
exceed the scope of the unicast prefix embedded in the multicast
It's no real surprise that this is difficult to answer as IPv6 connectivity is so rare that testing the Internet for IPv6 multicast ability and reliability is impossible for most end users, and as a result there are not a lot of articles written about it. Indeed, most end users have no idea why they would use IPv6 multicast at home, but the applications are ready and waiting.
This page talks about some confusion brought on by the RFCs, and RFC3956 mentions how certain multicast domains have trouble talking to each other. It may, at this point, be difficult to implement, but I see no reason why the game server (from my above example) cannot assign itself a multicast IPv6 address and inform the clients of this address, and all without having to beg for a static multicast IPv6 assignment.
This is something I would like to follow up myself in the future.
Step 1: ISPs need to enable IPv6. Still.
See also: RFC6308: Overview of the Internet Multicast Addressing Architecture