Out of the box, an instance will come up with IPv4. If you then perform DHCP for IPv6, you should still be able to route to the per-region archive mirrors (which only serve over IPv4 because when they were deployed IPv6 wasn't available).

If you disable IPv4 entirely, then it's true that you won't be able to access the archive mirrors. However, this isn't exactly "out of the box". :)

If you do want to disable IPv4, then at the same time you'll also need to modify your /etc/apt/sources.list to point at archive.ubuntu.com (which does serve over IPv6).

We're looking at how we can best make the "no IPv4" case work with the in-cloud mirrors, and hope to have that resolved soon.

After a complex set of steps to setup ipv6 in my [VPC, Router, Gateway, Security Groups, and others that I'm forgetting], I was able to launch 2 instances and get both ipv4 and ipv6 addresses. As OddBloke notes, I did have to 'sudo dhclient -6' to grab that ipv6 address in the instance. Once I did, I was able to ssh back and forth using ipv6 (yay!). Then I tried 'apt update'. In fact, the mirrors inside of AWS worked us fine. But security.ubuntu.com did not. It did resolve, but it didn't respond. Rest assured we (Canonical) are working this right now... Thanks.

I actually hit this problem, and I fixed it by modifying the routing table associated with my VPC to route ::/0 to an Egress Only Internet Gateway. (Just _adding_ the EIGW to a VPC is not sufficient, because routes aren't configured automatically.)

So I'm guessing that was the issue here.

And of course while it's Canonical's fault for not having a v6 dhcp, the similar problem affects Amazon Linux instances: repo.eu-central-1.amazonaws.com is ipv4-only so yum makecache fails.

Xenial's v6 dhcp has been working much better for me since this was fixed https://bugs.launchpad.net/ubuntu/+source/isc-dhcp/+bug/1633...

Prior to that dhclient fix, dhcpv6 was basically unusuable on 16.04.