IPv6 multicast addresses: Is the Group ID field effectively 112 bits or 32 bits?

Posted by Jeremy Friesner on Stack Overflow See other posts from Stack Overflow or by Jeremy Friesner
Published on 2009-12-23T18:15:03Z Indexed on 2010/04/28 0:13 UTC
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Hi all,

I'm trying to understand the rules for choosing an IPv6 multicast address Group ID, and the RFC seems somewhat inconsistent. For example, in RFC 2373 section 2.7 this diagram is shown:

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

... but then in section 2.7.2 it shows this:

|   8    |  4 |  4 |          80 bits          |     32 bits     |
+------ -+----+----+---------------------------+-----------------+
|11111111|flgs|scop|   reserved must be zero   |    group ID     |
+--------+----+----+---------------------------+-----------------+

So my question is, are the upper 80 bits of the Group ID field usable or not? If they are usable, is it only under certain circumstances (e.g. when using non-Ethernet networking technology?) What problems should I expect to experience if I set these bits when multicasting over an Ethernet LAN?

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