I think visitor's IP, browser, session, and cookie can be visitor indentifiers.
What else do you have for visitor indetifiers ?
Yes, by confirming the username and password, we make session, cookie or session and cookie at a same time.
I like to collect some information about visitors before he/she says he/seh is him/her.
Because sessions and cookies don't work across subdomain, I like to make a loose logIn keeping logic by combination of collected informations (IP and browser).
Thank you all the time ...