There are more than one possibility to tunnel IPv6 packets over IPv4-only links.
A point-to-point tunnel is a dedicated tunnel to an endpoint, which knows about your IPv6 network (for backward routing) and the IPv4 address of your tunnel endpoint and defined in RFC 2893 / Transition Mechanisms for IPv6 Hosts and Routers. Requirements:
IPv4 address of your local tunnel endpoint must be static, global unique and reachable from the foreign tunnel endpoint
A global IPv6 prefix assigned to you (see 6bone registry)
A foreign tunnel endpoint which is capable to route your IPv6 prefix to your local tunnel endpoint (mostly remote manual configuration required)
Automatic tunneling occurs, when a node directly connects another node gotten the IPv4 address of the other node before.
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
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.
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 220.127.116.11. In the background routing protocols handle this, see RFC 3068 / An Anycast Prefix for 6to4 Relay Routers for details.
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.