Port concept, what is a port In network technology, a port has two meanings: one is a physical port, such as the interface used by ADSL modems, hubs, switches and routers to connect other network devices, such as RJ-45 port and SC port. The second is logical port, which generally refers to the port in TCP/IP protocol. Port numbers range from 0 to 65535, such as port 80 for browsing web services, port 2 1 for FTP services and so on. What I want to introduce here is the logical port.

To view the port in Windows 2000/XP/Server 2003, you can use the Netstat command: click Start → Run, type "cmd" and press enter to open the command prompt window. Type "netstat -a -n" at the command prompt, and then press Enter to view the port number and status of TCP and UDP connections displayed in digital form.

Close/Open Ports How to close/open ports in Windows, because by default, many unsafe or useless ports are open, such as port 23 for Telnet service, port 2 1 for FTP service, port 25 for SMTP service, port 135 for RPC service, etc. In order to ensure the security of the system, we can close/open the port by the following methods.

1. To close a port, such as port 25 of SMTP service in Windows 2000/XP, you can do this: first open the control panel, double-click the administrative tools, and then double-click the service. Then find and double-click the Simple Mail Transfer Protocol (SMTP) service in the opened service window, click the Stop button to stop the service, then select Disable in the Startup Type, and finally click the OK button. In this way, closing the SMTP service is equivalent to closing the corresponding port.

2. Open the port If you want to open the port, just select Automatic in the startup type, click OK, then open the service, click Start Enable Port in the service status, and finally click OK. Tip: There is no "service" option in Windows 98. You can use the rule setting function of the firewall to close/open the port.

Functions of various ports: 0 Service: Reserved Description: Usually used to analyze the operating system. This method is effective because "0" is an invalid port in some systems, and when you try to connect it to a port that is usually closed, it will produce different results. A typical scan uses the IP address 0.0.0.0, sets the ACK bit and broadcasts it in the Ethernet layer.