64、 Network Addres Translation(NAT) is an Internet standard that enbles a local-area network to use one set of IP addresses for internet traffic and another set of ( ) IP addresses for external traffic. The main use of NAT is to limit the number of public IP addresses that an organization or company must use, for both economy and ( ) purposes. NAT remaps an IP address space into another by modifying network address information in the ( ) header of packets while they are in transit across a traffic routing device. It has become an essential tool in conserving global address space in the face of ( ) address exhaustion. When a packet traverses outside the local network, NAT converts the private IP address to a public IP address. If NAT runs out of public addresses，the packets will be dropped and ( )"host unreachable"packets will be sent.(1) A. localB. privateC. publicD. dynamic(2) A. politicalB. fairnessC. efficiencyD. security(3) A.MACB. IPC.TCPD. UDP(4) A. IPv4B. IPv6C. MACD. logical(5) A. BGPB. IGMPC. ICMPD. SNMP