There are more than one possibility to tunnel IPv6 packets over IPv4-only links.
6to4 tunneling (RFC 3056 / Connection of IPv6 Domains via IPv4 Clouds) uses a simple mechanism to create automatic tunnels. Each node with a global unique IPv4 address is able to be a 6to4 tunnel endpoint (if no IPv4 firewall prohibits traffic). 6to4 tunneling is mostly not a one-to-one tunnel. This case of tunneling can be divided into upstream and downstream tunneling. Also, a special IPv6 address indicates that this node will use 6to4 tunneling for connecting the world-wide IPv6 network
220.127.116.11. Generation of 6to4 prefix
The 6to4 address is defined like following (schema is taken from RFC 3056 / Connection of IPv6 Domains via IPv4 Clouds):
| 3+13 | 32 | 16 | 64 bits |
| FP+TLA | V4ADDR | SLA ID | Interface ID |
| 0x2002 | | | |
FP and TLA together (16 bits) have the value 0x2002. V4ADDR is the node's global unique IPv4 address (in hexadecimal notation). SLA is the subnet identifier (65536 local subnets possible) and are usable to represent your local network structure.
For gateways, such prefix is generated by normally using SLA "0000" and suffix "::1" (not a must, can be an arbitrary one with local-scope) and assigned to the 6to4 tunnel interface. Note that Microsoft Windows uses V4ADDR also for suffix.
18.104.22.168. 6to4 upstream tunneling
The node has to know to which foreign tunnel endpoint its in IPv4 packed IPv6 packets should be send to. In "early" days of 6to4 tunneling, dedicated upstream accepting routers were defined. See NSayer's 6to4 information for a list of routers.
Nowadays, 6to4 upstream routers can be found auto-magically using the anycast address 22.214.171.124. In the background routing protocols handle this, see RFC 3068 / An Anycast Prefix for 6to4 Relay Routers for details.
126.96.36.199. 6to4 downstream tunneling
The downstream (6bone -> your 6to4 enabled node) is not really fix and can vary from foreign host which originated packets were send to. There exist two possibilities:
Foreign host uses 6to4 and sends packet direct back to your node (see below)
Foreign host sends packets back to the world-wide IPv6 network and depending on the dynamic routing a relay router create a automatic tunnel back to your node.
188.8.131.52. Possible 6to4 traffic
from 6to4 to 6to4: is normally directly tunneled between the both 6to4 enabled hosts
from 6to4 to non-6to4: is sent via upstream tunneling
non-6to4 to 6to4: is sent via downstream tunneling