【71-75】Network Addres Translation（NAT） is an Internet standard that enbles a local-area network to useonesetofIPaddresses for internet traffic and another set of （71） IP addresses for external traffi The main use of NATis tolimitthenumber of public IP addresses that an organization or company must use， for both economy and （72） purposes. NATremapsan IP address space into another by modifying network address information in the （73） header of packets whiletheyareintransit across a traffic routing device. It has become an essential tool in conserving global address space inthefaceof（74）address exhaustion. When a packet traverses outside the local network， NAT converts the private IP address toapublicIPaddress. If NAT runs out of public addresses，the packets will be dropped and （75）"host unreachable" packets will besent.
