first of all, you need to know how the client uses it, otherwise you can only guess roughly. From this point of view, it should be a random port. If the client is also made with netty, then you can be sure that it is xx.connect (ip,port). There is no specified port here, and at the same time, you need to send network information through socket. It is usually sent by selecting an unused port (the port is very large), and then shutting down to release the port