Final answer:
Windows 7 and Windows 8 have Teredo built-in, which facilitates IPv6 connectivity through IPv4 networks. OS X and Linux do not have Teredo support by default.
Step-by-step explanation:
The question asks which operating systems have Teredo built-in, with the options being Windows 7, OS X, Windows 8, and Linux. Teredo is a networking protocol designed to allow IPv6 connectivity through IPv4 network address translators (NATs), which is useful for transitioning from IPv4 to IPv6.
Windows 7 and Windows 8 have built-in support for Teredo. Microsoft has been supportive of IPv6 and has included Teredo in these versions of Windows to help users connect to IPv6 resources even if their local network is still using IPv4.
In contrast, OS X and Linux do not have built-in support for Teredo by default, though it can be implemented on Linux systems through third-party software.
The operating systems that have Teredo built-in are Windows 7 and Windows 8. Teredo is an IPv6 transition technology that allows IPv6 packets to be transmitted over an IPv4 network. It provides a way for devices using IPv6 to communicate with devices using IPv4.