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ipv6 Aggregatable global unicast addresses

Last updated on December 26, 2018

        Studying IPv6 addressing I haven't understood IPv6 aggregable global unicast addresses, I mean on  <a href="http://tools.ietf.org/html/rfc4291">RFC 4291</a> there is a generic scheme:
|         n bits         |   m bits  |       128-n-m bits         |
+------------------------+-----------+----------------------------+
| global routing prefix  | subnet ID |       interface ID         |
+------------------------+-----------+----------------------------+
But looking for formats used I see in my book(Halsall) the format used is this:
|  3  | 5  |  8   |    32          |       16      |          64                    |
+-----+----+------+----------------+---------------+--------------------------------+
| 010 | reg| TLA  |  NLA           |  SLA          |  interface ID                  |
+-----+----+------+----------------+---------------+--------------------------------+
and in the web, for example on the microsoft site, the format used is this:
|  3  |     13    |     8    |       24      |    16     |      64                  |
+-----+----+------+----------+---------------+-----------+--------------------------+
| 001 |    TLA    | reserved |    NLA        |    SLA    |        interface ID      |
+-----+----+------+----------+---------------+-----------+--------------------------+
My questions are:
  • why is there more than one format of aggregatable global unicast addresses?
  • Are there other formats or these 2 are the only 2 defined?
Thanks.

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