Nmap

Nmap (Network Mapper) is a free and open source utility for network discovery and security auditing. It has a lot of serious tools for auditing.

Supports host discovery, has multiple port scanning methods (TCP SYN, connect, UDP and others), identification of port-based applications and their versions. Detects operating systems and firewall filtering, and runs NSE scripts that check hosts for known vulnerabilities.

Has flexible options for specifying target devices and ports.

Nmap helps administrators find live hosts, open ports and running services, making security audits of a network much faster.

Works from the command line; the Windows installer also adds the Zenmap graphical interface, Ncat, Ndiff and the Npcap packet capture library.

It has received "Information Security Product of the Year" awards from Linux Journal and Info World. Nmap has also appeared in many popular movies, including "The Matrix Reloaded", "Die Hard 4" and "The Bourne Ultimatum".