Nmap: The Network Mapper
Nmap's design allows system administrators and curious individuals to scan large networks to determine which hosts are up and what services they are offering. Nmap supports several scanning techniques such as UDP, TCP connect(), TCP SYN (half open), FTP proxy (bounce attack), Reverse-ident, ICMP (ping sweep), FIN, ACK sweep, Xmas Tree, SYN sweep, and Null scan. (See the Scan Types section for more details.)
Nmap also offers a number of advanced features, such as remote OS detection via TCP/IP fingerprinting, stealth scanning, dynamic delay and retransmission calculations, parallel scanning, detection of down hosts via parallel pings, decoy scanning, port filtering detection, direct (non-portmapper) RPC scanning, fragmentation scanning, and flexible target and port specification.