An open port scanner is a tool which is used to check the external IP address and identify open ports on the connection. Port scanning itself is not illegal, but scanning the destination host without authorization is illegal and you will get into trouble. Port scanners help the server administrators and penetration testers to examine at which ports the data is entering into the network and to protect from invaders. The most common security practice is to close unused ports in private machines, in order to block known access to any service which may keep running on the PC.”]
Source: https://gbhackers.com/open-port-scanner-port-checker-tools/