说明：  内网的计算机以NAT（网络地址转换）协议，通过一个公共的网关访问因特网。内网的计算机可向因特网上的其它计算机发送连接请求，但因特网上的其它计算机无法向内网的计算机发送连接请求，所以无法用通常的方法去建立FTP服务器
(Computers within the network to NAT (Network Address Translation) protocol, through a common gateway to access the Internet. Computer within the network to the Internet can send connection requests to other computers, but not other computers on the Internet computer network to send incoming connection requests, it is not in the usual way to build an FTP server)