Urgent. For ftp active and passive, hope you can answer this question !!

Urgent. For ftp active and passive, hope you can answer this question !! -- Linux general technology-Linux technology and application information. For details, refer to the following section. On the ftp server, if both the client and the server are in the internal network, the server only performs port 21 ing on the route. Why can I access the client in both active and cooperative modes: neither the server nor the client is a dmz host. After both the server and the client are connected to the nat and firewall, the data port is coordinated by the command port during the establishment of the data port, this does not perform nat translation on the route. If one party directly accesses the data port of the other party, the route will lose the data packet because there is no NAT ing, and the data port cannot be connected, why? What do you expect?