map-block algorithm?

Hi,

Can someone please explain to me the algorithm used by the map-block rule to
allocate addresses/ports for eg:
map-block tun0 10.0.0.0/8 -> 20.20.20.0/24 auto

Is the allocation always deterministic? ie, am I able to based on the
natted address/port combination work out what the original source address
was.

Based on what I have read thus far the "auto" parameter seems to suggest
that each natted ip will have 252 ports available for use, whereas the
"ports 64" suggest that each natted ip will have only 64 ports available for
use - is this the case?

Thanks,
Kwang