Angry IP Scanner
Angry IP Scanner is a free open source network security application developed in Java by Anton Keks for Linux, Macintosh, and Windows. It is a tool to help network administrators find unknown devices, vulnerabilities, and bottlenecks in their networks. The program works by sending requests using various protocols to every address in the range of IPs you specify and finding which addresses respond to the various requests and how long it takes them to respond. With this information, a network administrator can see which computers may be at risk. The program has been around since at least 2006.
Personal
I found this program while looking for a way to determine if my network was secure when I was living in an apartment with hundreds of other wired and wireless networks around me. Using it, I discovered all sorts of unsecured devices, including some of my own. I still use it to get the IP of devices that connect to my home network, but don't have an interface which can allow me to easily get it.
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