Ok, I think now I got it. What confused me, was the fact, that you don't
need further authentication, when you logged in a windows system.
Now my next question. Is it possible to use the windows authentication
for a different user, than the User, that I'm logged in?
For example I log in on a windows system with the user WinUserA and
password abc. Is it then possible to connect to a firebird database with
the windows user WinUserB with password xyz?

Regards

Guido