A port scanner is an application designed to probe a server or host for open ports. Such an application can be used by administrators to verify security policies on their networks and by attackers to identify network services running on a host and exploit vulnerabilities.

A port scan or portscan is a process that sends client requests to a range of server port addresses on a host, with the goal of finding an active port; this is not a nefarious process in and of itself. The majority of uses of a port scan are not attacks, but rather simple probes to determine services available on a remote machine.