r1 ipv6 confusion by default

hi there, just created the account to report what i experienced with my r1.
unfortunately i had a bunch of changes w/o reboot so i only can estimate what caused the gap but im pretty sure it was at least the fact that i changed to the /etc/network/interfaces.r1router cfg before i rebooted from do-release-upgrade to xenial (install image has been the recent trusty of today). that was when i couldnt even ping my banana from any other box in my home net.
have to say im with the german telekom which provides ipv6 in a dual stack manor.
i saw on the pi console that it got the exspected ipv4, but also ipv6 addresses by telekom speedport dhcp. nevertheless from my other box it looked like that there has not been another ipv4 computer on that address while i could see on the pi cli it had been assigned ...
i connected from the other box by using an ipv6 i read on tty of the banana which succeeded.
after all my quick'n dirty solution was to follow this guide to simply disable ipv6, after reboot ipv4 ssh from other boxes was working again.

this brought me to the conclusion i indeed have to deal with ipv6 conversion of my personality - but have mercy - not today

but i thought this information could be usefull for you since i appreciate your work.

cheers ...