The port number at the transport layer can only be recognized by two peer layers. What is the internal connection between this and the network layer? Add your own source port number and destination port number to the transport layer, give them to the network layer (not to mention the lower two layers), then hand them over to the other host through the network, take off the frame shell and give it to the network layer, and the network layer CRC, check that the destination IP is its own, take off the IP packet and pass it to the transport layer, and check the port number to the application. When connecting to a specific application, the destination port number is fixed. For example, the FTP port number is 2 1 and Telnet is 23. The port number that sends a connection request to the same host must be different, because it has to identify and divide various data processing. If you want to know where the destination IP address comes from, it is DNS.