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 NAT

71.