Internet. In practice, when a Teredo client wants to contact a native IPv6 node, it must locate the corresponding Teredo relay, i.e., to which public IPv4 and UDP port number to send encapsulated IPv6 packets. To do that, the client crafts an ICMPv6 Echo Request (ping) toward the IPv6 node, and sends it through its configured Teredo server. The Teredo server de-capsulates the ping onto the IPv6 Internet, so that the ping should eventually reach the IPv6 node. The IPv6 node should then reply with an ICMPv6 Echo Reply, as mandated by RFC 2460. This reply packet is routed to the closest Teredo relay, which — finally — tries to contact the Teredo client. Maintaining a Teredo server requires little bandwidth, because they are not involved in actual transmission and reception of IPv6 traffic packets. Also, it does not involve any access to the Internet routing protocols. The only requirements for a Teredo server are: The ability to emit ICMPv6 packets with a source address belonging to the Teredo prefix Two distinct public IPv4 addresses. Though not written down in the official specification, Microsoft Windows clients expect both addresses to be consecutive — the second IPv4 address is for NAT detection Public Teredo servers: teredo.trex.fi (Finland) Former public Teredo servers: teredo.remlab.net / teredo-debian.remlab.net (Germany), now redirects to teredo.trex.fi Relays A Teredo relay potentially requires much network bandwidth. Also, it must export (advertise) a route toward the Teredo IPv6 prefix (2001::/32) to other IPv6 hosts. That way, the Teredo relay receives traffic from the IPv6 hosts addressed to any Teredo client, and forwards it over UDP/IPv4. Symmetrically, it receives packets from Teredo clients addressed to native IPv6 hosts over UDP/IPv4 and injects those into the native IPv6 network. In practice, network administrators can set up a private Teredo relay for their company or campus. This provides a short path between their IPv6 network and any Teredo client. However, setting up a Teredo relay on a scale beyond that of a single network requires the ability to export BGP IPv6 routes to the other autonomous systems (AS's). Unlike 6to4, where the two halves of a connection can use different relays, traffic between a native IPv6 host and a Teredo client uses the same Teredo relay, namely the one closest to the native IPv6 host network-wise. The Teredo client cannot localize a relay by itself (since it cannot send IPv6 packets by itself).