With this, the ISP could prevent the user from running ND proxy, so implementations will likely have a knob which would ignore this bit, but still..
Since such an ISP would violate the "assign a /48 by default" recommendation, it might as well go all the way and only assign a /128 to the subscriber.
So why would the ISP use the more cumbersome approach of 1) assiging a /64, and 2) lying by setting the P-bit in its RA?
Because if such an ISP used RAs for address assignment, it couldn't restrict to /128. (Well, maybe such ISP's would require stateful DHCPv6 assignment..)
-- Pekka Savola "You each name yourselves king, yet the Netcore Oy kingdom bleeds." Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings
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