What is Port Scanning / Testing
In modern day networking, the Transport Layer of the Internet Protocol Suite, like Transmission Control Protocol (“TCP”) and the User Datagram Protocol (“UDP”), use a numerical identifier for the data structures of the endpoints for host-to-host communications. Such an endpoint is known as a port and the identifier is the port number. The Internet Assigned Numbers Authority (IANA) is responsible for maintaining the official assignments of port numbers for specific uses.
The complete list of these Port Numbers is available at Wikipedia.
Port Scanning is the technique to probe a network host for open ports. This is often used by administrators to verify security policies of their networks and by attackers to identify running services on a host with the view to compromise it. To portscan a host is to scan for listening ports on a single target host.