According to this article
http://www.consulintel.euro6ix.org/ietf/draft-palet-v6ops-6in4-vs-6over4-01.html
6in4 is only encapsulation mechanism that is used by transition mechanisms like 6over4.
What kind of tunnel is
ip tunnel mode sit ?
I found on many sites that it is 6in4 or 6over4. 6in4 is incorrect according to this article. 6over4 uses only multicast addresses but this command allows use other ip addresses.