Teredo and 6to4
Teredo (RFC 4380) and 6to4 (RFC 3056) are IPv6 transition mechanisms that tunnel IPv6 packets inside IPv4 traffic, letting a device reach the IPv6 internet even when its own network only provides IPv4 -- Teredo encapsulates packets in UDP specifically so the tunnel can traverse most NAT devices, while 6to4 wraps them directly in IPv4 protocol 41 and expects a public IPv4 address at the tunnel endpoint. Both predate today's widespread native dual-stack deployment and are now considered legacy: Microsoft disabled Teredo by default starting with Windows 10, and RFC 7526 formally deprecated 6to4's anycast relay infrastructure. The practical risk is that a tunnel operating inside otherwise-allowed UDP or protocol-41 traffic can give a device an IPv6 address a firewall configured only with IPv4 rules in mind was never written to cover.
By AdminKit team, Netbase JSC · Updated