Nmap

Nmap: Uses raw IP packets in novel ways to determine what hosts are available on the network.

Nmap (Network Mapper) is a free security scanner designed for exploration of computer networks or security auditing.

Nmap can discover hosts and services on a network by sending raw IP packets to a target host and through the analysis of the responses it can determine running services, operating systems, the type of packet filters/firewalls and many other characteristics.

Nmap is perfectly capable of scanning large networks at high speed but works just as well for single hosts.