Network Address ( 71) (NAT) is the process where a network device assigns a public address to a computer (or group of computers) inside a private network. 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 ( 72 ) purposes.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 ( 73 ) "host unreachable '’ packets will be sent. Multiple private IP addresses are mapped to a pool of public IP addresses in (74)NAT. In contrast, many private IP addresses can also be translated to a single public IP address，and (75 ) are used to distinguish the traffics. This is most frequently used as it is cost effective as thousands of user scan be connected to the Internet by using only one public IP address.

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