A port scanner tells you what’s on your network by scanning the network to see what’s alive and working. Port scanners give basic views of how the network is created. They can help identify unauthorized hosts or apps and network host configuration failures that can cause dangerous security flaws.
The big picture from port scanners usually shows security issues that might otherwise go unseen. Port scanners are simple to use and can test network hosts regardless of what operating systems and apps they are running. The tests are often completed almost quickly without having to affect individual network hosts, which would be a real pain otherwise.
The method of evaluating your overall network security is understanding the results you get from a port scan. You can get false positives on open ports, and you might also have to go deeper. For instance, UDP scans — like the protocol itself — are less reliable than Transmission Control Protocol (TCP) scans and usually generate false positives because many apps don’t know how to respond to random incoming UDP requests.
Sometimes port scans can take a long time. The length of time relies on the number of hosts you have, the number of ports you scan, the tools you use, the processing power of your test system, and the speed of your network links.