> [RFC4862] requires that subnets operating stateless address
> autoconfiguration use 64 bit prefixes,
Actually, not true! Stateless address autoconfiguration supports
different prefix lengths just fine. That was a deliberate design
decision, even after we switched to 64-bit Interface IDs. E.g (from
4862):
A link-local address is formed by combining the well-known link-local
prefix FE80::0 [RFC4291] (of appropriate length) with an interface
identifier as follows:
1. The left-most 'prefix length' bits of the address are those of
the link-local prefix.
2. The bits in the address to the right of the link-local prefix are
set to all zeroes.
3. If the length of the interface identifier is N bits, the right-
most N bits of the address are replaced by the interface
identifier.
If the sum of the link-local prefix length and N is larger than 128,
autoconfiguration fails and manual configuration is required. The
length of the interface identifier is defined in a separate link-
type-specific document, which should also be consistent with the
address architecture [RFC4291] (see Section 2). These documents will
carefully define the length so that link-local addresses can be
autoconfigured on the link.
and so on.
> and [RFC4291] requires that interface identifiers conform to
> modified EUI-64 format.
That is where the effective requirement is hard coded in.
Thomas
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