NMAP (Network Mapper) is a free and open source utility for network discovery and security auditing. It is used to scan network to live systems and ports/services on the systems which helps network admins, system admins to get inventory and build infrastructure (Network + Systems + Services) topology.
Nmap can do the following scans:
IP Sweep Scan – Scans for Live IP addresses in the network.
Port Scan – Scans for open ports on a system in the network.