Understanding ISATAP

By default, the IPv6 protocol in Windows 7 automatically configures a link-local unicast IPv6 address of the form FE80::5EFE:w.x.y.z (for private IPv4 addresses) or FE80::200:5EFE:w.x.y.z (for public IPv4 addresses). This address is a link-local ISATAP address, and it is assigned to the ISATAP tunneling interface. Using their link-local ISATAP addresses, two ISATAP hosts (such as computers running Windows 7) can communicate using IPv6 by tunneling across an IPv4- only network infrastructure (such as a network whose routers forward only IPv4 packets and not IPv6 packets).

Note In Windows 7 and in Windows Vista SP1 or later versions, link-local ISATAP addresses are automatically configured only if the name "ISATAP " (the ISATAP router name) can be resolved. Otherwise, the ISATAP interface will be media disconnected. However, if you administratively enable ISATAP by using the netsh interface isatap set state enabled command, the link-local address will be configured regardless of whether the ISATAP router name can be resolved.

With the addition of one or more ISATAP routers (IPv6-enabled routers that advertise address prefixes, forward packets between ISATAP hosts and other ISATAP routers, and act as default routers for ISATAP hosts), a variety of transition topologies become possible, including:

• Connecting ISATAP hosts on an IPv4-only intranet to an IPv6-capable network.
• Connecting multiple "islands" of ISATAP hosts through an IPv6-capable backbone.

These configurations are possible because ISATAP routers advertise address prefixes that enable ISATAP hosts (such as computers running Windows 7) to autoconfigure global or unique local unicast IPv6 addresses.

Note Without the presence of an ISATAP router, ISATAP hosts running Windows Vista RTM could only autoconfigure link-local unicast IPv6 addresses, which limited IPv6 communications to those between hosts on the IPv4-only intranet. This was changed in Windows Vista SP1 so that without an ISATAP router, the interface will show media disconnected. In other words, Windows Vista SP1 won't configure a link-local ISATAP address when no ISATAP router is configured. The behavior in Windows 7 is the same as in Windows Vista SP1.

Note For more information on how ISATAP works, see the white paper, "IPv6 Transition Technologies," at http://www.microsoft.com/downloads/details.aspx?FamilyID=afe56282-2903-40f3-a5ba-a87bf92c096d &displaylang=en.