dhcpcd DHCPv6 Prefix Delegation - help making a prefix

[Roy Marples <roy%marples.name@localhost>]

Hi List

dhcpcd (in my git tree) supports DHCPv6 Prefix Delegation and has done 
for a while now.
It's undergone extensive testing and I feel that it's ready aside from 
one issue - configuration.

Currently we get a prefix delegated and we don't know it's length in 
advance.
Realistically it will be from /48 to /64.

Here are some examples:
3FFE:FFFF:5::/48
3FFE:1234:5678:fc::/62

This is a problem as the user is expected to supply an SLA and length 
to match the delegated prefix and generate a /64.
Also, this has to be divisble by 8 with my current code (i think).

So assuming we have a unique number per interface how can we generate a 
/64 for each interface and prefix automatically so that all the user as 
to do is request a PD and optionally interfaces to apply it to?

Thanks

Roy