user authentication

Hi,
I do not understand something in PostgreSQL Security and I hope
someone will help me.
I've installed PostgreSQL 7.1.3 on FreeBSD 4.7
I made a paswword for user postgres but when I just do the following
command:
# psql -U postgres dbname
PostgreSQL do not even ask me for password.
The same thing happens when I try to connect to database via network.
So anyone who wants can make everything with my database!
May be there are some configuration files that I should change?
But why by default PostgreSQL is such unsafe?

Thanks.
--
Best regards,
 Andrei Malashevich                          mailto:andy@chg.ru