Identifying location by host IP number

Hi,
To what extent is it possible to positively identify the geographical 
location of a visitor to one's site by analysing environment 
variables, and notably host IP numbers?

I have long assumed it was pretty well impossible to tell what part 
of the world a given user was coming from, but someone recently told 
me that blocks of IP numbers were distributed on a country basis, and 
that it therefore was possible if you had the right info.

Is this true, and if so where can I find the relevant data?

Many thanks for any help.
-- 
--
David Sharp, journaliste, France http://www.sharp-words.com/
Tel (home) 331 42 64 35 94  - (office) 331 40 41 47 92
  E-mail <david.sharp(at)sharp-words.com>	ICQ: 16881741

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