The Interface Identifier of an IPv4 interface using 6over4 is the 32-bit IPv4 address of that interface, padded to the left with 0s and is 64 bits in length. Note that the Universal/Local bit is 0, indicating that the Interface Identifier is not globally unique. When the host has more than one IPv4 address in use on the physical interface concerned, an administrative choice of one of these IPv4 addresses is made.

The IPv6 Link-local address for an IPv4 virtual interface is formed by appending the Interface Identifier, as defined above, to the prefix FE80::/64.

Figure 1. 6over4 address format

Global unicast addresses are generated by adding a 64-bit prefix to the 6over4 Interface Identifier. These prefixes can be learned in any of the normal ways, for example, as part of stateless address autoconfiguration or by way of manual configuration.

6over4 is a transition mechanism which allows isolated IPv6 hosts, located on a physical link which has no directly connected IPv6 router, to use an IPv4 multicast domain as their virtual local link. A 6over4 host uses an IPv4 address for the interface in the creation of the IPv6 interface ID, placing the 32-bit IPv4 address in the low-order bits and padding to the left with 0's for a total of 64 bits. The IPv6 prefix used is the normal IPv6 prefix, and can be manually configured or dynamically learned by way of Stateless Address Autoconfiguration.

Because 6over4 creates a virtual link using IPv4 multicast, at least one IPv6 router using the same method must be connected to the same IPv4 multicast domain if IPv6 routing to other links is required.

When encapsulating the IPv6 packet, the source IP address for the IPv4 packet is an IPv4 address from the sending interface of the 6over4 host. The destination IPv4 address is the low-order 32 bits of the IPv6 address of the next-hop for the packet. Note that the final destination of the packet does not need to be a 6over4 host, although it might be one.