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       看看维基百科的解释， 太棒了：

       Network address translation (NAT) is a methodology of modifying  information in  (IP)  packet headers while they are in transit across a traffic  for the purpose of remapping one IP  into another.

   意思是说：nat会改变IP数据包。

   Most often today, NAT is used in conjunction with IP masquerading which is a technique that hides an entire IP address space, usually consisting of  IP addresses (), behind a single IP address in another, usually public address space. This mechanism is implemented in a routing device that uses  translation tables to map the "hidden" addresses into a single IP address and readdresses the outgoing Internet Protocol packets on exit so they appear to originate from the routing device. In the reverse communications path, responses are mapped back to the originating IP addresses using the rules ("state") stored in the translation tables. The translation table rules established in this fashion are flushed after a short period unless new traffic refreshes their state.

   意思是说：局部pc一直对外。 外部无法获知具体的局部信息。

    The method enables communication through the router only when the conversation originates in the masqueraded network, since this establishes the translation tables. For example, a  in the masqueraded network can browse a website outside, but a web browser outside could not browse a web site hosted within the masqueraded network. 

    意思是说： 外部无法首先与内部沟通， 因为没有转换表， 无法沟通。 必须是内部先与外部沟通。

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