6to4 is a transition mechanism that allows modern IPv6 sites to communicate over a legacy IPv4 network, like The Internet. Just like other auto-tunneling techniques the IPv6 end-to-end connectivity is made possible by encapsulating the IPv6 datagrams inside IPv4 datagrams.
Auto-tunneling is a last resort method compared to dual-stack or native IPv6 support, but it can be used as a temporary solution for providing IPv6 connectivity. Just don’t expect kick-ass performance http://www.potaroo.net/ispcol/2010-12/6to4fail.html.
