Nmap is a utility for network exploration or security auditing. It
supports ping scanning (determine which hosts are up), many port
scanning techniques, version detection (determine service protocols
and application versions listening behind ports), and TCP/IP
fingerprinting (remote host OS or device identification). Nmap also
offers flexible target and port specification, decoy/stealth scanning,
sunRPC scanning, and more.

WWW: http://www.insecure.org/nmap/

See the web page and the Phrack Magazine article (Volume 7, Issue 51 
September 01, 1997, article 11 of 17) http://www.insecure.org/nmap/p51-11.txt

-- David
   obrien@cs.ucdavis.edu