Since a little while back apt-get has started to try using ipv6 connections, these do not work on our network (they are always 101 unreachable)
Why does it try them and how do I stop it from trying them?
Why does it not rollover and try another connection/mirror?
It's the only thing that does and it is a royal pain in the butt.
Today it's the damn mythic-beasts one.
There are some old posts on the internet I can see, but I know you lot known it w.r.t. Raspian a lot better.
(Fully updated Raspian on Pi's)
That is before I go trying random solutions.
Various number of 'apt-get update' eventually knocks it onto an ipv4 connection.