Microsoft has made available an update for Windows 7 and Windows Server 2008 R2 to improve the operability and performance for these operating systems when you migrate from IPv4 to IPv6. Specifically the update resolves an issue where clients with a public IPv4 address, which are automatically assigned a 6to4 IPv6 address, may not be able to reach IPv6 hosts. The update includes a feature that allows the client to check and verify end-to-end IPv6 connectivity through the 6to4 relay before adding the IPv6 route to the routing table. This addresses one of the IPv6 “brokenness” issues where clients would try to establish an IPv6 connection to an address that could not be reached through the relay. The update also addresses stability issues when many IPv6 addresses and routes are used, and alters the default behavior of Internet Connection Sharing with 6to4. In addition, when clients are configured to use IPv6 as their default connection but don’t have an IPv6 connection to the Internet, the update enables the use of the Network Connectivity Status Indicator (NCSI) functionality to verify IPv6 Internet connectivity before establishing a connection. If an IPv6 connection to the Internet is not available, the client will use IPv4 instead of IPv6.
You can download the IPv6 readiness update for Windows 7 and Windows Server 2008 R2 here.