Nmap ("Network Mapper") is a free and open source (license) utility for network exploration or security auditing. Many systems and network administrators also find it useful for tasks such as network inventory, managing schedules upgrade service, and monitoring host or service availability.
Nmap uses raw IP packets in new ways to determine what hosts are available on the network, what services (application name and version) those hosts are offering, what operating systems (and OS versions) they use, what type of packet filters / firewalls are in use, and dozens of other characteristics. It was designed to rapidly scan large networks, but works fine on a single target.
nmap.org
