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As I understand it, public IPv6 addresses allocated by the IANA will have the prefix 2000::/3. These ip addresses will be routable through the Internet.

IPv6 multicast addresses on the other side are prefixed FF00::/8.

So my understanding is that the ipv6 multicast addresses won't be routable through the internet. I am right? If so, is there any way to do one-to-many ip routing over the Internet in IPv6?


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I might not have explained myself correctly. What I meant was that the ipv6 multicast addresses are not considered like public addresses. 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? – Absolom May 29 '12 at 21:33

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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.

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Yes but I don't see IANA allocating chunk of multicast addresses like they do for public IP addresses. Doesn't it mean that you cannot reserve public multicast addresses and therefore you wouldn't be able to route multicast addresses over the Internet without the risk of having address clashing? – Absolom May 29 '12 at 21:58
@Absolom, the second link I added in my comment included a list of the permanent IANA IPv6 assignments. It can get a dynamic address using a protocol like DHCP ( or apparently it can just get an address by picking a random number. all 3 options are basically the same as what was available with IPv4. – Zoredache May 29 '12 at 23:52
thanks for the clarifications – Absolom May 30 '12 at 12:33
On IPv4 networks connected to the mbone global multicast works. Mbone consists mostly of university networks. Making available interesting video services advertised via SAP or for example NTP. There are security issues: Clients for ghost installs have shown up from 8 timezones away until work blocked that specific multicast group at the border. – Koos van den Hout Jun 2 '12 at 14:59
I think the major issue providers have with global multicast is that it requires a bunch of state inside the network, so it just doesn't scale up to Internet size. Also, figuring out who/how to bill for the traffic amplification is probably tricky in a large service provider network. – rmalayter Jun 4 '12 at 21:10

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.

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But will the ipv6 multicast addresses will be routable over the internet? Say, for example, you want to stream a live video feed over the internet to a ipv6 multicast group. Will this be supported? Or the ipv6 multicast will only work in private networks? – Absolom May 29 '12 at 21:29
Yes - some ranges of the FF00::/8 will be routable globally, others will have limitations. One of the fields defines administrative scope, which gives routers a clue about how far to forward a given multicast. I'll add an example to my answer. – rnxrx May 29 '12 at 21:37

The standards allow global multicast routing, but AFAIK currently most ISPs restrict multicast only for their use (IPTV etc.)

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Could you please direct me to the standard that you are referring to? – Absolom May 29 '12 at 21:49
2 ffxe::/16 are multicast addresses with global scope, so reachable globally. – mulaz May 29 '12 at 22:10

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.

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You confuse two things in my opinion: multicast routing and IPv6 address allocation.

Multicast routing depends on all routers on the path between the multicast source and you forwarding multicast packets to the destinations which want to receive a certain multicast group. In an uncontrolled network with full multicast network any source system can send to any multicast group without any controls, and all systems which have joined that group will receive the data. This has nothing to do with any allocation, just with multicast routing being enabled.

IPv6 address allocation from 2000::/3 decides who can use which unicast address.

So who can use which IPv6 multicast group is for as far as I know only defined in rfc3307.

Multicast sources do not have a lot of control which destinations see their packets, it is quite possible data transmitted via multicast reaches destionations not available via unicast.

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yes I understand the difference but I maybe I didn't ask the question correctly. What I was implying with my question was that IANA is allocating global unicast IPv6 addresses to avoid address clashing. They don't do the same thing for the multicast addresses, so I thought that multicast was not intended to be routed over the Internet. Zoredache mentioned something about generating random multicast addresses to avoid this problem. – Absolom Jun 4 '12 at 17:46

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