nmap is a utility for port scanning large networks, although it works fine
for single hosts. The guiding philosophy for the creation of nmap was TMTOWTDI
(There's More Than One Way To Do It).  Sometimes you need speed, other times
you may need stealth. In some cases, bypassing firewalls may be required. Not
to mention the fact that you may want to scan different protocols (UDP, TCP,
ICMP, etc.). You just can't do all this with one scanning mode.  Thus nmap
incorporats virtually every scanning technique known of.

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

See the webpage 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