simple web server Internet database refers to a network connection server built on a physical server, always waiting for client requests and responding. The communication between the client and the server supports the HTTP protocol Yes, the client can be any program that supports the HTTP protocol Before the client sends an HTTP request to the server, first establish a TCP connection, send an HTTP request through a TCP connection, the client and server establish a TCP connection using socket (socket) Plug: Can be understood as an abstraction of a connection endpoint, allowing a program to communicate with another program through a file descriptor. TCP socket pair: 4-tuple: local ip, local port, external ip, external port