IPv6

My (new) ISP provides Ipv6.
I configured a M0n0 and things work. (although some sites seem to have an awful response via ipv6  )

Question: First of, I did not enter an site-level aggregator ID and expected it to be 0. It turned out to use 1 default? Why?

Explicitly entering 0 does what I want, but I'm curious.

Wish: The system uses ::EUI as LAN interface address. Would it be possible to define some other address (like ::ffff which I can remember )