Port scanner is a tool or a software application that scans a network or an individual host for open ports.

Port scanners have a good use and a bad use depending upon the individual who is using them.

In the hands of an administrator, port scanners help identify and verify security issues through open ports.

In the hands of a hacker, a port scanner would help to find (and hack) the services that are running on your computer.

Port scans are normally referred in a negative way and seen as a malicious activity on a single port or a range of ports with the ultimate goal of finding a vulnerable service on an open port that might be exploitable.

Port sweeping is another term that is linked with port scanners and is described as a method of scanning multiple hosts for a specific service or a port.

There are several port scanning types:
•    TCP Scan
•    SYN Scan
•    UDP Scan
•    ACK Scan
•    Window Scan
•    FIN Scan

And some more unusual types:
•    XMAS Scan
•    NULL Scan
•    IDLE Scan
•    ICMP Scan
•    PROXY Scan

The legal implications of port scan are very grave and it is considered an illegal activity.

Make sure that you have prior written permission from the target network’s administrator or system owner before you proceed for a port scan.

Administrators use port scanning for auditing the security of their network, verifying the compliance with security policies of the network, etc. There are many tools that are available for port scanning. The most famous of them all is nMap that is available freely at nmap.org.