Among the ports used by the server, well-known applications must be assigned well-known port numbers, and applications without well-known port numbers must be registered to avoid duplication, so they need to be divided into three types. The function of the port is to enable the application processes of computers running different operating systems to communicate with each other. Compare the IP address to a house, and the port is the door of the house. A real house has only a few doors, but an IP address can have as many as 65536 ports (2 16)! Ports are marked with port numbers, which are only integers between 0 and 65535 (2 16- 1). The service function of each network is different, and different data packets need to be sent to different services for processing. When the host starts FTP and WWW services at the same time, the sent data packet will be sent to FTP service or WWW service according to the port number on TCP.