What is Teredo?
Windows Server 2008 includes built-in support for IPv6. In fact, just like Windows Vista and Windows 7, IPv6 is enabled on Windows Server 2008 by default. As businesses migrate from the current version of IP, which is IPv4, to the newer version IPv6, they can take advantage of some of the transition technologies, such as Teredo.
What is Teredo? It is an IPv6 transition technology that provides translation between IPv4 and IPv6 clients by encapsulating the IPv6 packets in IPv4 UDP packets. Because it encapsulates IPv6 packets within IPv4 UDP datagrams, IPv6 packets can be routed on IPv4 Internet and across Network Address Translation (NAT) devices. The use of IPv4 NATs is pretty common for Internet connectivity in a lot of Small Office/Home Office (SOHO) configurations. Teredo provides automatic tunneling of IPv6 packets across the IPv4 Internet for hosts that are located behind the IPv4 NATs. One of the biggest advantage of Teredo over 6to4 tunneling protocol is that you don't need to upgrade your NAT devices because Teredo encapsulates IPv6 packets within IPv4 UDP packets that can be forwarded by most current NATs.
NATs and Teredo
Speaking of NATs, there are three major types of NAT devices in use today: Cone, Restricted, and Symmetric. To understand how Teredo functionality, it is helpful to have some basic understanding of the types of NATs. If you are planning on implementing IPv6 and utilize Teredo in your network, perhaps the first question you are going to ask is "Will Teredo work with my NAT device?" As long as your NAT device supports UDP port translation, it will support Teredo, unless you are using Symmetric NAT. I won't go into the details as to which version of Windows works with which type of NAT and under what scenario but generally speaking, Teredo will work well with a Cone NAT or a Restricted NAT.
NAT devices store a NAT translation table, similar to a routing table in a router. The translation table contains either dynamic or static mappings, just like a router contains dynamic and static entries. A Cone NAT keeps a mapping (or translation table) of internal address and port number and an external address and port number. This allows traffic to flow between any internal source address and port number to the external address and port number. A Symmetric NAT on the other hand has a mapping of the same internal address and port number to a different external address and port number. Teredo doesn't work with this type of mapping. A Restricted NAT has a mapping between an internal address and port number and an external address and port number but the mapping is for either specific source address and port numbers, or for specific source addresses.
The Teredo technology consists of 4 major components, or nodes, that work together. Before you deploy Teredo, make sure that these nodes are in place and that you have met all the necessary requirements for deploying a Teredo solution on your network.
Teredo Client: A Teredo Client is a host connected to the Internet using IPv4 from behind a NAT and uses the Teredo tunneling protocol to access the IPv6 Internet. A Teredo Client is included with Windows Server 2008, Windows 7, Windows Vista, Windows Server 2003 SP1 and SP2, Windows XP SP2, Windows XP SP1 with Advanced Networking Pack for Windows XP.
Teredo Server: The Teredo Server is used for the initial configuration of Teredo tunnel and for address configuration of Teredo clients. None of the Teredo Clients listed above include a Teredo Server. However, Microsoft has deployed Teredo Servers on the Internet that will work with these clients. You can contact Microsoft to obtain an update for the Teredo Server functionality. When using Teredo, a lot depends on the type of NAT that you are using. For example, a Cone NAT will require a use of both a Teredo Relay and a Teredo Server in your IPv6 network.
Teredo Relay: A Teredo Relay acts as an IPv6/IPv4 router that forwards packets between Teredo clients on the IPv4 Internet and the IPv6-only hosts. The Teredo Clients listed above do not include a Teredo Relay functionality. Your best hope is to find an Internet Service Provider (ISP) that offers a Teredo Relay so your Teredo Clients can take advantage of that functionality.
Teredo Host-Specific Relay: This component is an IPv6/IPv4 node that connects to both the IPv4 Internet and the IPv6 Internet. It has the ability to communicate with the Teredo Clients directly across the IPv4 Internet without needing a Teredo Relay. The Teredo Clients listed above include a Teredo Host-Specific Relay functionality and if the computer has a global address assigned to it then this functionality is enabled by default. If the global address is not assigned to the computer then the Teredo Client functionality is enabled.
NOTE: The Teredo Server, Teredo Relay and the Teredo Host-Specific Relay listen for Teredo traffic on UDP port 3544 so make sure that your routers and firewall allow traffic on that port to pass through. Also, keep in mind that when you facilitate communication between an IPv4 and IPv6 network through a Cone NAT, a Teredo Relay and a Teredo Server is required in an IPv6 network, not an IPv4 network.
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