Which functionality is required within an IP router that is situated at the boundary of an  IPv4 network and an IPv6 network to allow communication between IPv6- only and IPv4 - only nodes?（）

• A、Autoconfiguration
• B、Automatic 6to4 Tunnel
• C、Automatic 6to4 Relay
• D、Network Address Translator - Protocol Translator (NAT - PT)
• E、Intrasite Automatic T unnel Address Protocol (ISATAP)