71-75、NAT is a kind of transition technology which can be used to alleviate IPv4 address shortage.When a packet sent by an（71）user for accessing the Internet reaches a NAT-enabled gateway,the gateway translates the Ip address in the（72）header of the packet to another Ip addressand the port number to another port number,and then forwards the packet to the（73）In thisprocess the gateway can translate the Ip addresses of packets from different intranet users tothe same public IP address and differentiate intranet users based on（74）.In this way,the（75）IP address can be reused.Based on whether source or destination IP addresses of packetsare translated,NAT is classified into source NAT,destination NAT and bidirectional NAT71.
internal
Internet
ethernet
intranet
这段英文描述了NAT（特别是NAPT/PAT）的工作原理。当一个来自内部网络(intranet)(71)的用户访问互联网时，NAT网关会修改IP数据报(IP datagram)(72)的头部，将其源IP和源端口号转换为一个公共(public)(75)IP地址和另一个端口号，然后将数据包转发到互联网(Internet)(73)。网关通过记录端口号(port numbers)(74)的映射关系来区分不同的内部用户会话。