Keith,
>I am not sure I fully understand the distinction between reserved and
>unassigned. It would seem to me that in neither case is it safe to use one
>of the values for any purpose whatsoever, since a reserved value might
>subsequently become unreserved, and an unassigned value might subsequently
>become assigned. In either case, any intermediate use will lead to a
>potential encoding clash.
Correct.
>I see that the IPX class has been replaced by unassigned, also some other
>previous classes like geographic and provider-based have also been removed.
The geographic and provider-based were removed sometime ago when the
current document (RFC2373) was published and was replaced by the
Aggregatable Global Unicast Addresses format. The IPX block was removed in
the current draft after checking that there no intended usage planned.
>This seems to imply that these schemes are not seen as being used in the
>foreseeable future, and even their use as previously specified might lead
>to an encoding clash at some later date. Presumably these values could be
>used for some other completely different scheme in the future, unrelated
>to their previous incarnation.
>
>Is my interpretation and conclusion correct?
The currently published document, RFC2373, in the IETF standard definition
of the IPv6 address blocks. When the current draft is published as an RFC
it will obsolete RFC2373. Any use of the address blocks outside of the
current assignments are likely to conflict with future assignments and
would be outside of IETF defined usage.
>On the other hand, I could see how these values might say reserved rather
>than unassigned. If a value is reserved is interpreted as never will be
>assigned, then it would be safe to use in its original context. But if
>this were the case why not leave them in as before?
Reserved means that the block is intended for a particular usage or should
not be used. Unassigned means that it is open for future definition and
usage. The IPX block was changed from reserved to unassigned because it
was no longer going to be used for IPX and is now available for future
definition.
Hope this helps.
Regards,
Bob
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