It's up to you how you assign your addresses. You can do pretty much anything you want.
I (whether this is right or wrong) have assigned both interfaces eth0 (LAN) and ppp0 (broadband) with the same IPv6 address (the first address of the /48 block they assigned me). Eg. if my prefix is 2001:5:6::/48, I've slapped eth0 and ppp0 with 2001:5:6::
Whilst this works, I've recently ran into some problems with some transparent proxying I'm trying to setup and I think it's because I have put the same address on both interfaces. If you want, you can set the WAN with 2001:5:6:ffff:: and leave the LAN with 2001:5:6:0:: which separates the two addresses and removes any confusion the router may have for having the same address on two interfaces (I may go this route shortly with what I'm trying to do).
Also, I read somewhere about the pros and cons of using the very first
available address (2001:5:6:: rather than 2001:5:6::1) - can't remember where I read that now, but I'm sure there is documentation online about it.
FYI, when you enable IPv6 (which is an option that has to be explicitly enabled on Linux PPP setup), AA only assign you link level IPv6 address to your WAN interface - nothing more. See my PPP log below:
Nov 19 01:23:09 axis pppd: CHAP authentication succeeded: BBEU01234567
Nov 19 01:23:09 axis pppd: CHAP authentication succeededNov 19 01:23:09 axis pppd: local IP address 90.155.X.X
Nov 19 01:23:09 axis pppd: remote IP address 81.187.X.XNov 19 01:23:09 axis pppd: local LL address fe80::f919:7bf2:5d89:8ff3
Nov 19 01:23:09 axis pppd: remote LL address fe80::0203:97ff:fe16:c000
These are the addresses assigned (and they change at each PPP login). It is then up to you (as I said above) to configure your interfaces and routing as you wish under the /48 prefix that's routed towards your connection.
Edited by mixt (Tue 20-Nov-12 13:50:14)