The reason is that computer communication uses port numbers to identify different applications or services. When a program or service starts listening on a port number, it will bind to that port and enter listening state. This means that it will accept incoming connection requests and establish communication connections with other computers or devices.
Extended content:
1. Port number range: the range of port numbers is from 0 to 65535, where 0 to 1023 are some commonly used port numbers reserved for specific services, called "well-known ports", such as port 80 of HTTP and port 443 of HTTPS. And 1024 to 49 15 1 are the registration ports of some common applications. 49 152 to 65535 are dynamic or dedicated ports that can be temporarily assigned to applications.
2. The role of monitoring: The monitoring port allows programs or services on the computer to communicate with other devices or applications. Network services can be provided through listening ports, such as Web server listening port 80 accepting HTTP requests and FTP server listening port 2 1 accepting file transfer requests.
3. Unmonitored port: When the port is not monitored by any program or service, it is in listening state. This means that there is no application waiting for a connection request on this port, so a communication connection with this port cannot be established. The port that is not listening may be because the related service is not running, the service is not started or blocked by a firewall.
It should be noted that just because a port is not listening does not mean that the port is closed or unavailable. Some ports may be blocked by firewalls or not allowed to be accessed from outside, but they can still be used for internal communication or specific applications.