example.com and www.example.com are treated as different sessions

example.com and www.example.com are treated as different sessions

So i have a small website that requires users to log in with a username
and password. Everything works fine apart from 1 tiny problem
when the user logs in using the url studeez.net, he or she is redirected
to a page where the top bar displays his Firstname, lastname and an option
to logout.
But if i add www. before the link so that it becomes www.studeez.net, the
user is given the login /register options.
This is quite the issue because when the user tries to log in, he or she
gets a feedback that they are already logged in.
How do i harmonize studeez.net and www.studeez.net as well as all the
subsequent urls like studeez.ney/path/to/page and
www.studeez.ney/path/to/page