Common ports of TCP protocol are: FTP (file transfer protocol, using port 2 1. Download files, upload home pages, all need FTP service), Telnet (a port of remote login, users can remotely connect to the computer as their own identity, through which they can provide a communication service based on DOS mode. For example, the previous BBS was a pure character interface, and the servers supporting BBS used 23 ports to provide services to the outside world. ), SMTP (defines a simple mail transfer protocol, using 25 ports. ), POP3(POP3 corresponds to SMTP. POP3 is used to receive mail, using port 1 10. )