"In this tutorial I'm using a Debian server in a LAN
(i.e., it uses a router to connect to the Internet) with the
ethernet device eth0 and the IPv4 address 192.168.0.100. The
router's public IP address at the time of this writing was
18.104.22.168. With IPv6, other systems can connect directly to the
Debian system, regardless of the router and NAT. This tutorial will
work for Debian systems that are connected directly to the Internet
(i.e., no NAT, no router) as well.
"To use IPv6, we will configure a tunnel that connects our IPv6
Debian system to IPv6 hardware on the other end (run by a so-called
"tunnel broker") and thus to the IPv6 backbone. This tunnel is
necessary because most ISPs don't support direct IPv6 connectivity,
and it doesn't make sense to route IPv6 traffic over an IPv4
network because chances are that the next-hop router doesn't know
what to do with this traffic."