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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?

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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.

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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? Oct 9, 2015 at 9:38
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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. Oct 9, 2015 at 9:52
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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 9, 2015 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, Oct 9, 2015 at 12:10
  • None are "available". At best, they're "unassigned". Use IPv6 multicast the way it was designed.
    – womble
    Oct 11, 2015 at 23:28

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