http://signal.eu.org/blog/2007/12/12/ipv6-chez-free/ is informative
too. (this is also in french).
rfc4291 says:
For all unicast addresses, except those that start with the binary
value 000, Interface IDs are required to be 64 bits long and to be
constructed in Modified EUI-64 format.
so if you further divide a /64 into multiple subnets you aren't
compliant with the standards, and you break the usual ipv6 address
autoconfiguration method.
the current _guidelines_ (not requirements) for allocations to
subscribers are given in rfc3177:
[...]
- /48 in the general case, except for very large subscribers
- /64 when it is known that one and only one subnet is needed by
design
- /128 when it is absolutely known that one and only one device is
connecting.
In particular, we recommend:
- Home network subscribers, connecting through on-demand or
always-on connections should receive a /48.
- Small and large enterprises should receive a /48.
- Very large subscribers could receive a /47 or slightly shorter
prefix, or multiple /48's.
- Mobile networks, such as vehicles or mobile phones with an
additional network interface (such as bluetooth or 802.11b)
should receive a static /64 prefix to allow the connection of
multiple devices through one subnet.
- A single PC, with no additional need to subnet, dialing-up from
a hotel room may receive its /128 IPv6 address for a PPP style
connection as part of a /64 prefix.
Note that there seems to be little benefit in not giving a /48 if
future growth is anticipated. In the following, we give the
arguments for a uniform use of /48 and then demonstrate that it is
entirely compatible with responsible stewardship of the total IPv6
address space.
[...]
there are some suggestions out (see recent nanog posts) that /56 be
used for private consumer subscribers, though this is probably not
useful for all but the largest consumer ISPs.
