Port scanning is basically the process of connecting to a target computer and sending a message to each port. Depending on the kind of response received, the user doing the port scanning can determine if the port is accessible or closed. Free port scanner software includes downloadable applications, web-based applications offered as a service, or a feature in a larger application that maps out networks.
1. Understand what ports are.
Ports are virtual connections in a network used by programs to directly exchange data. Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two common methods of network communication that use ports. They are assigned numbers for identification, and each type of protocol uses ports numbered from 0 to 65535. TCP ports numbered from 0 to 1023 are called Well-Known Ports, as they have been associated over the years with standard services such as File Transfer Protocol (FTP) and Hypertext Transfer Protocol (HTTP). Ports numbered 1024 and above are not commonly associated with standard services, and are separately called Registered Ports (from 1024 to 49151) and Dynamic or Private Ports (from 49152 to 65535). In operating port scanner software, you will need to choose which type of ports you want probed, and then based on the software's evaluation you will need to decide which ones to close down and which to keep open.
2. Be careful in performing a port scan.
Port scanner applications can be used to test your own system's security. In such cases there would be no legal issue, as you're only performing it on your own home network. If it's part of auditing an office's network security, you will need permission from higher-ups. Port scanners are essentially a reconnaissance tool, and carrying a scan out can easily be mistaken for intrusion. Do not scan a system or a computer if the user, owner or administrator of which did not give consent.
3. Run the port scanner.
If you plan to use a downloadable application, then you need to install it in your system first. When you run the application from your PC, you will be asked to enter the target computer, choose which type of ports and which kind of port scanning you wish to do. With web-based port scanners the process is much simpler. Navigate to the website and follow the outlined steps. Your computer or IP address will be automatically detected, and then you will be also asked to choose which ports and what type of scanning you wish to execute. In both cases, you will receive a report after the scan, which will inform you what ports in your system or the targeted computer are open, and will offer recommendations on how to better secure them.
Port scanners can be used to help boost system or network security, or it can be used to compromise it. It all depends on the intentions of the one doing the port scanning.