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What Is Port Scanning?

Port scanning is the process of probing a device or server to find out which ports are open, closed, or filtered. Since each port corresponds to a specific service or application, scanning them gives a picture of what a system is running and where it may accept incoming connections. 

Every device connected to the internet communicates through numbered ports, ranging from 0 to 65535. Port scanning sends requests to these ports and analyzes the responses. An open port replies and signals an active service. A closed port responds but rejects the connection. A filtered port gives no response, often because a firewall is blocking it. 

Network administrators use port scanning routinely to audit systems, verify configurations, and spot vulnerabilities before bad actors do. Security tools like Nmap are built specifically for this purpose. However, unauthorized port scanning on systems you don’t own can be considered a hostile or illegal act depending on the jurisdiction.