In the hopes that it saves someone else two hours later: the ISP Astound only supports IPv6 in Washington State. You might find this page which says “Astound supports IPv6 in most locations”. Their tech support agents might tell you that they support v6 on your connection, even if you are not in Washington. “Yes, we do support both DHCPv6 and SLAAC”, they might say, and tell you to use a prefix delegation size of 60. If you are staring at tcpdump
and wondering why you’re not seeing anything coming back from your router’s plaintive requests for address information, it is because they do not, in fact, support v6 anywhere but Washington.
I asked RCN what their plan was for IPv6 back in 2010 or so and they said they just… didn’t have a plan. At least they were honest, I guess?
(RCN has since been bought by Astound, so perhaps they have the concepts of a plan now. :-P)