单选题 Network Address Translation (NAT)is an Internet standard that enables a local-area network to use one set IP addresses for internal traffic and another set of（ 此空作答 ）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 of both, 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.