This IP Access Control module allows you to easily assign IP addresses (or ranges) from which users may log in.
Simply put, you let in who you want, and block out every one else from even seeing your site. The ultimate in web protection.
You might use this feature for example if you have customer service reps or other personnel that you don't want to log in from home. You can assign your internal network IP range at the office as acceptable login locations for all users, but then not add any remote IP addresses except for maybe the admin user and a supervisor.