单项选择题

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)