6over4 is an IPv6 transition mechanism meant to transmit IPv6 packets between dual-stack nodes on top of a multicast-enabled IPv4 network.
6over4 defines a trivial method for generating a link-local IPv6 address from an IPv4 address, and a mechanism to perform Neighbor Discovery on top of IPv4.
6over4 relies on IPv4 multicast availability which is not very widely supported by IPv4 networking infrastructure.
6over4 is of limited practical use, and is not supported by the most common operating systems.
ISATAP is a more complex alternative to 6over4 which does not rely on IPv4 multicast.