http://de.wikipedia.org/wiki/6to4
http://www.ietf.org/rfc/rfc3056.txt

works like this:

You have a host in the IPv4 internet, and you want it to become a host
in the IPv6 internet.

You construct your host's globally unique IPv6 address (actually, 2^80
globally unique IPv6 addresses in a /48 IPv6 network) from its
globally unique IPv4 address like this:

IPv6 network address = 2002:[the IPv4 address]

e.g. IPv4 address = 100.200.100.200 =>
     IPv6 network address = 2002:64c8:64c8::/48

(0x2002 is 16 bits, your IPv4 address has 32 bits, IPv6 addresses have
128 bits, so 80 bits remain)

(The 0x2002 is the "well-known" prefix reserved for IPv6 hosts that
connect to the IPv6 internet via the 6to4 protocol)

For sending an IPv6 packet to a desired target IPv6 host, you
construct the packet with target address=that host's IPv6 address
(call it T), source address=any IPv6 address from your IPv6 network as
constructed above.

You then wrap this IPv6 packet as payload into an IPv4 packet, as
specified by 6to4 (RFC 3056), and send it via IPv4 to one of the
public "6to4 gateways" (those are hosts that live in the IPv4 internet
as well as in the IPv6 internet). The gateway unwraps your IPv6 packet
and sends it on over the IPv6 internet to T.

When the T host sends an IPv6 (response) packet to you, it will
construct that packet with target address=your 6to4 IPv6 address. This
packet will at some point reach a 6to4 gateway (not necessarily the
same one that previously sent on the packet originating from you),
which will wrap the packet into an IPv4 packet and send it via IPv4 to
your host, which unwraps the IPv6 packet again and delivers it to the
local IPv6 stack.

List of public 6to4 gateways:

http://www.kfu.com/~nsayer/6to4/

As a measure of convenience, there is the special, well-known anycast
IPv4 address 192.88.99.1, which routers should automatically forward
to the nearest 6to4 gateway (see RFC 3068) so you won't have to pick a
6to4 gateway by hand.