Discovery, also known as Host Discovery, is a process to enumerate live hosts and is a very important component of the reconnaissance phase of a security testing activity. This will help you to eliminate the unwanted hosts from the list of targets, thus it will allow you to use these enumerated hosts to perform targeted scans and penetration tests.  Some of the tools that can be used to perform Network Discovery are Nmap, Nessus, OpenVas, and Wireshark.

The following screenshot shows a sample host scanned using Nmap for Discovery. It shows that the host is up, thus we can determine the host is live:

These tools come in handy if the ping is disabled across the network. I always prefer using Nmap over other tools because of its ease of use and the Nmap Script Engine (NSE), which allows the user to write and implement custom scripts. We will be discussing NSE in coming chapters.

In this course, we will further introduce you to various recipes on how to perform host discovery manually and using tools.