IPv6 Tunnel behind Thomson modem

Took me a little while to figure out, but I found how to create a point-to-point IPv6 tunnel (cordially provided by SixXS) if you're behind a Thomson modem. In my case, it's a ST546, but I imagine this works for other stuff too. WARNING: I'm NOT entirely sure what I'm doing here exactly. IF IT BREAKS YOUR MODEM, IT'S YOUR OWN FAULT!!! Consider yourself warned.

If you still dare, follow this:

• Telnet to your modem and login with your Admin credentials
• Perform the command: nat maplist. You'll see something like: {Administrator}=>nat maplist Idx Type Interface Outside Address Inside Address Use 1 NAPT Internet 12.345.678.90:22 192.168.1.200:22 1 2 NAPT Internet 12.345.678.90:113 192.168.1.200:113 0 3 NAPT Internet 12.345.678.90:49163 192.168.1.201:5060 1 4 NAPT Internet 12.345.678.90:49205 192.168.1.202:5060 2 5 NAPT Internet 12.345.678.90:49238 192.168.1.203:5060 2 6 NAT Internet 12.345.678.90 127.0.0.1 0 {Administrator}=> The Interface is important.
• We're going to add a mapping for the 6to4 protocol by giving it the command: nat mapadd intf=Internet type=nat outside_addr=12.345.678.90 inside_addr=192.168.0.1 protocol=6to4. Replace "Internet" with your interface name, the outside address with the outside address (IPv4) of your modem and inside_address with the internal IP address of the server that you're going to use for the connection.
• Now give the command saveall
• IMPORTANT: I had to ping out before I was able to receive any 6to4 packages! So ping6 ipv6.google.com and see if the outside world can ping6 your server.

That worked for me. I had a lot of trouble finding this solution, so I'm blogging about it in the hope that it helps someone else. The most important info was found in a post from Richard Menedetter on the SixXS forum. It was hard to find for me, though.