Teredo — Free

Teredo provided a necessary, albeit complex, solution for IPv6 connectivity in the era of pervasive NAT. Its main contribution is demonstrating that UDP encapsulation can successfully traverse NATs for layer-3 protocols. However, due to native IPv6 growth and security liabilities, Teredo is now considered a legacy technology. Network engineers should prefer native IPv6, 6to4 where applicable, or application-layer proxies.