A port scanner is an application designed to probe a server or host for open ports. Such an application may be used by administrators to verify security policies of 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.

To portsweep is to scan multiple hosts for a specific listening port. The latter is typically used to search for a specific service, for example, an SQL-based computer worm may portsweep looking for hosts listening on TCP port 1433.

The design and operation of the Internet is based on the Internet Protocol Suite, commonly also called TCP/IP. In this system, network services are referenced using two components: a host address and a port number. There are 65535 distinct and usable port numbers, numbered 1 … 65535. (Port zero is not a usable port number.) Most services use one, or at most a limited range of, port numbers.

Some port scanners scan only the most common port numbers, or ports most commonly associated with vulnerable services, on a given host.

The result of a scan on a port is usually generalized into one of three categories:

Open ports present two vulnerabilities of which administrators must be wary:

Filtered ports do not tend to present vulnerabilities.