IPv6 addresses from the 2002::/16 prefix must be used with 6to4 tunneling. The 32 bits following the 2002::/16 prefix correspond to the IPv4 address of the tunnel source. For example, if the IPv4 address of the tunnel source is 10.14.177.1, you could use 2002:0A0E:B101::/128 for the IPv6 address of the tunnel interface; the hexadecimal value 0A corresponds to the decimal value 10, the hexadecimal value 0E corresponds to the decimal value 14, the hexadecimal value B1 corresponds to the decimal value 177, and the hexadecimal value 01 corresponds to the decimal value 1.

Although an IPv6 unicast address can be used with the IntraSite Automatic Tunnel Addressing Protocol (ISATAP) tunneling method, only IPv6 addresses with the 2002::/16prefix can be used with 6to4 tunneling.
IPv6 packets are encapsulated inside an IPv4 packet that has a protocol type of 41, not aprotocol type of 42. When an IPv4 packet with a protocol type of 41 arrives on the router interface, the IPv6 packet is decapsulated and mapped to the appropriate IPv6 tunnel interface based on the IPv4 address.

Connectionless Network Service (CLNS) cannot be used with 6to4 tunneling. CLNS packet scan be encapsulated using Generic Routing Encapsulation (GRE) and IPv4compatible tunneling. GRE and IPv4 compatible tunneling can encapsulate a variety of Network layer packets, including IPv6.

Because 6to4 tunneling is an automatic, point-to-multipoint tunneling method, a tunnel destination is not required. Other point-to-multipoint tunneling methods include ISATAP and IPv4compatible tunneling. GRE and manual tunneling are point-to-point tunneling methods that require an IPv4 address for the tunnel destination.