Teredo

Teredo is a transition technology that provides IPv6 connectivity to devices on an IPv4 network that is separated from the IPv6 Internet by a NAT. It tunnels IPv6 packets inside UDP over IPv4, allowing hosts behind NAT to communicate with other IPv6 hosts even when native IPv6 connectivity is not available.

A Teredo client behind a NAT contacts a Teredo server to obtain a Teredo IPv6 address. The system uses a network of Teredo clients, servers, and relays. The client receives an address with the prefix 2001:0000::/32, encoding information about the chosen server and the client's NAT state, plus a random identifier. IPv6 packets are encapsulated in UDP and sent to a Teredo relay, which forwards them to the native IPv6 Internet.

Teredo was described in early IPv6 transition specifications and has been implemented in major operating systems, notably Windows. Its operation depends on reachable Teredo servers and relays and on the behavior of the client's NAT. Some NAT types and firewalls can block Teredo, and performance depends on network conditions. Privacy and security considerations include potential leakage of local network information and the possibility of abuse to bypass certain firewall policies.

Today Teredo is one of several IPv6 transition mechanisms. With increasing native IPv6 deployment, its use has declined, and many networks disable Teredo by default or rely on alternatives such as native IPv6, 6to4, or DS-Lite.