Teredo Tunneling

NAT has been used to overcome ipv4 address shortage as it acts as an interface between Internet and the local network. It translates private address of local network to the public address of ISP. Now that ipv6 is a reality, overcoming the ipv4 to ipv6 transition is an important issue. This issue was resolved to certain extent by 6to4 protocol, that could be implemented on NAS.
Teredo Tunneling is a protocol that gives ipv6 connectivity to nodes that connect through NAT devices that do not understand ipv6. In this protocol, ipv6 packets are encapsulated into ipv4 UDP datagrams that can route through NAT and ipv4 Internet. Teredo was introduced because 6to4 protocol, that is most common ipv6 over ipv4 protocol, was economically and technically difficult to implement on most NAT devices. Teredo in turn is implemented on nodes itself that can easily understand the encapsulated packets and thus gain ipv6 connectivity without NAT being ipv6 aware.