DISTINCT to get distinct *substrings*?

Hello to the list,

here's an SQL question, I hope it's not off topic. From a list of
URLs I want to get only the distinct values of the *web sites* these
URLs belong to, that is everything before and including the 3rd
slash, and I think this should be possible within the DB. I would
like to say something like

SELECT substring(attribute from '^http://[^/]*/') from pg_atp where
attribute like 'http://%'

(which works) but get only the distinct values. SELECT DISTINCT ON
substring.. doesn't work. Probably I haven't understood the semantics
of the DISTINCT keyword. Can anybody help?

thanks in advance
Christoph