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There are couple of differences between IPv6 and IPv4 address allocation.
         The prefix length for an IPv6 subnet will always be /64; no more,
no less. It allows you to place as many IPv6 devices as the underlying
network
         medium allows.
         With IPv4, prefix length varies between subnets to subnets, and
it caused painful costs when renumbering subnets (for example, imagine
when you
         renumber an IPv4 subnet from /28 to /29 or vice versa).
         An ordinary leaf site will always get /48 of address space. It
will make site renumbering easier, and allows you to switch ISP more
easily.
         With IPv4, the allocation varies by the size of the site, and
made it very painful when you migrated from one ISP to another, for
example.
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"prefix length for an IPv6 subnet will always be /64".

does it mean that i MUST do /64 subnet for every ethernet segment or it
won't work?

and does it mean i MUST have last 64 bits of address equal to function of
ethernet MAC address.

or can i do whatever i want (with route and specyfying net-prefix
manually) and all above is only most often/proposed use?


i'm sure i can get only /64 not /48 address space so it's important