title: IPv6 multicast addresses: Is the Group ID field effectively 112 bits or 32 bits?
body: <p>I'm trying to understand the rules for choosing an IPv6 multicast address Group ID, and the RFC seems somewhat inconsistent.  For example, in <a href="https://www.rfc-editor.org/rfc/rfc2373#section-2.7" rel="nofollow noreferrer">RFC 2373</a> section 2.7 this diagram is shown:</p>
  <pre><code>|   8    |  4 |  4 |                  112 bits                   |
  +------ -+----+----+---------------------------------------------+
  |11111111|flgs|scop|                  group ID                   |
  +--------+----+----+---------------------------------------------+
  </code></pre>
  <p>... but then in section 2.7.2 it shows this:</p>
  <pre><code>|   8    |  4 |  4 |          80 bits          |     32 bits     |
  +------ -+----+----+---------------------------+-----------------+
  |11111111|flgs|scop|   reserved must be zero   |    group ID     |
  +--------+----+----+---------------------------+-----------------+
  </code></pre>
  <p>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?</p>