这种NAT内部的机器A连接过外网机器C后,NAT会打开一个端口.然后外网的任何发到这个打开的端口的UDP数据报都可以到达A.不管是不是C发过来的-such NAT internal machinery A connecting machines over external networks C, NAT will open a port. Then external networks to any of the open ports of UDP datagrams can be reached A. . whether or not it is sent to the C.