By default, any host can access Manager/Central Manager from any IP address. You can allow access to specific hosts by enabling GUI Access from Manager → <Admin Domain Name> → Setup menu and defining the list of authorized hosts/networks.
Note
You need to have at least one authorized host to enable GUI Access.
All attempts by authorized and unauthorized hosts to access your Manager are logged in the user activity log, which you can access from the View User Activity Audit Log link in the page.