NMap

This is an open source network sniffing tool that is commonly used to map networks. The tool records IP packets entering and leaving a network. It also maps out fine details about a network, such as the devices connected to it and also any open and closed ports. The tool can go as far as identifying the operating systems of the devices that are connected to the network, as well as the configurations of firewalls. It uses a simple text-based interface, but there is an advanced version of it called Zenmap that also has a GUI. Following is a screenshot of the nmap interface. The command being executed is:

#nmap 192.168.12.3 

This command is executed to scan the ports of the computer on the IP address 192.168.12.3: