Transition Mechanisms - Teredo Tunnelling

Teredo is a tunnelling technology invented to solve the problem of IPV4 network address translation (NAT).  NAT does not work well with tunnelling technologies.  Since a large number of users on the Internet sit behind a router with a private IP address developers had to come up with a solution to do this.  NAT does not allow forwarding of many types of payload within the IPV4 header.  Since IPv6 is the payload of the IPv4 header this is bound to cause problems.  6to4 tunnelling supports NAT as long as the router is running 6to4 and NAT in the same router.  NAT was designed to enable more users to connect to the Internet using the same IPv4 address.  This helped a lot with IPv4’s struggle to assign addresses to the ever growing Internet.  An IPv6 network will no longer need this type of technology, but since it is very widely deployed Teredo is a temporary solution to this problem.  Teredo works on UDP.  Since Teredo was developed to support NAT it has to possess unique global IPv4 addresses.  This technique is host-to-host based, which means it is only concerned with end-to-end connections.  The global IPv6 prefix for Teredo is 2001:0000:/32.  The address contains both the Teredo server and client address.  To route IPv6 packets using Teredo, a client needs to send a packet to a Teredo server.  The Teredo server then forwards the pack to a Teredo relay, the packet is then sent to the destination node. The 16 bit flag field holds information on the type of NAT that is needed to bypass. 

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