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.