Now, you are familiar with how to scan IP addresses and identify a live host within a subnet. In this section, we will discuss the services that are running on a host. These services are the ones that are using a network connection. A service using a network connection must open a port; from a port number, we can identify which service is running on the target machine. In pentesting, the significance of port scanning is to check whether an illegitimate service is running on the host machine.
Consider a situation where users normally use their computer to download a game, and a Trojan is identified during the installation of the game. The Trojan goes into hidden mode; opens a port; sends all the keystrokes, including log information, to the hacker. In this situation, port scanning helps to identify the unknown services that are...