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)