IPv6 multicast addresses: Is the Group ID field effectively 112 bits or 32 bits?
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by Jeremy Friesner
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Published on 2009-12-23T18:15:03Z
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2010/04/28
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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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