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How many freely available multicast prefixes are there? The RFC specifies every ff00::/8 as the required prefix. Accordingly, there are 16*16 = 256 multicast prefixes. However, a large part are already taken (e.g., ff02::2 ).

So how many are there actually available?

1 Answers1

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Your question doesn't make a lot of sense... there's no concept of "prefixes" in IPv6 multicast. The base RFC defines an IPv6 multicast address as looking like this:

|   8    |  4 |  4 |                  112 bits                   |
+------ -+----+----+---------------------------------------------+
|11111111|flgs|scop|                  group ID                   |
+--------+----+----+---------------------------------------------+

This means that, within each multicast scope, there are only 2^112 (5,192,296,858,534,827,628,530,496,329,220,096) multicast groups available (not many, but I'm sure you'll agree it's significantly more than 256).

There are a few "well-known" multicast addresses are available in an IANA-managed register. There are also ways to generate source-specific multicast addresses.

Of course, in proper IPv6 style, all of this is complicated somewhat with various options and flags, however all of the addressing schemes described therein provide 2^32 (about four billion) multicast group IDs per unicast /64 (typically called "a subnet" in IPv6). I think that should be enough for anyone in the nearish future.

womble
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  • What I mean is, in theory you with ffxx you have 256 available multicast addresses, discounting the group ID. But, for instance, ff02 is reserved for Link-local addresses. So, discounting those, how many ffxx are truly available? – Sven Akkermans Oct 09 '15 at 09:38
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    No, there is a maximum of 256 available combinations of flags and scopes. Each of those has 2^112 available addresses. Which flags and scope to use (the xxx part) depends on what you want and what the RFCs specify. – Sander Steffann Oct 09 '15 at 09:52
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    `ff02::/16` isn't "reserved for link-local addresses", it's the link-local *multicast scope*. A very, very different thing. The question "how many ffxx are truly available" is a question that I cannot make any sense out of. Available for *what*? Does it even make any sense to "count" them, and treat them as fungible, given that each combination of flags and scope has a distinct and very different meaning? – womble Oct 09 '15 at 10:09
  • Available in the sense that they are not used for anything else. For example, FF3E is used for nothing (as far as I know), while FF02 is, – Sven Akkermans Oct 09 '15 at 12:10
  • None are "available". At best, they're "unassigned". Use IPv6 multicast the way it was designed. – womble Oct 11 '15 at 23:28