The port can be a USB port, a COM serial port or an external connection port of a switch or router device of a hardware device. For software, port is the interface between communication protocols, which defines the services provided by UNIX or WINDOWS NT system. The local system will assign the corresponding port number, that is, the protocol port number, to the provided service process.

The protocol port number is identified by a positive integer ranging from 0 to 65535. Ports between 0- 1023 are well-known ports. These ports are generally assigned to some services, such as 2 1 port assigned to FTP service, 23 port assigned to TELNET, and 80 and 80 ports assigned to HTTP service. The ports between 1024 and 65535 are dynamically allocated, and these ports are generally not fixed to a service, which means that many services can use these ports. As long as the running program applies to the system for network access, the system can allocate one of these port numbers to the program, and the occupied port number will be released after the program process is closed.

According to TCP/IP protocol, there are two kinds of ports: TCP port and UDP port.

TCP port, that is, transmission control protocol port, is a connection-oriented service port, which needs to establish a connection between the client and the server to provide reliable data transmission. Common ports are 2 1 port of FTP service, 23 port of Telnet service, 25 port of SMTP service and so on.

UDP port, that is, user datagram protocol port, is a non-connection oriented service port, and there is no need to establish a connection between the client and the server. Common ones are port 53 for DNS service, port 69 for TFTP service, ports 8000 and 4000 for QQ, port 5060 for SIP and so on. The following are common protocol ports: