Editing Teredo tunneling (section)

=== Servers ===
Teredo clients use Teredo servers to autodetect the kind of NAT they are behind (if any), through a simplified STUN-like ''qualification procedure''. Teredo clients also maintain a binding on their NAT toward their Teredo server by sending a UDP packet at regular intervals. That ensures that the server can always contact any of its clients—which is required for [[NAT hole punching]] to work properly.

If a Teredo relay (or another Teredo client) must send an IPv6 packet to a Teredo client, it first sends a ''Teredo bubble'' packet to the client's Teredo server, whose IP address it infers from the Teredo IPv6 address of the Teredo client. The server then forwards the ''bubble'' to the client, so the Teredo client software knows it must do hole punching toward the Teredo relay.

Teredo servers can also transmit ICMPv6 packet from Teredo clients toward the IPv6 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&nbsp;— finally&nbsp;— 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&nbsp;— 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<ref>{{cite web|url=https://www.remlab.net/miredo/news.shtml.en|title=Miredo : News|author=Rémi Denis-Courmont|date=5 June 2021|access-date=2022-07-17|quote=For the time being, teredo.remlab.net will alias the public Teredo server provided by the TREX regional exchange in the Finnish city of Tampere.|archive-date=2022-07-17|archive-url=https://web.archive.org/web/20220717233659/https://www.remlab.net/miredo/news.shtml.en|url-status=live}}</ref>