In HTTP application, the server program does not know or need to know the host address and port number of the client program, and the server will passively wait for the HTTP request from the client to establish a connection. Therefore, customers must know the IP address and port number of the server. The IP address can be obtained by domain name resolution, and the port number of the server adopts the well-known port 80. After the server is started, it will listen to the client's connection request at port 80, so 80 is the port used by the server process in the HTTP protocol.