To restrict where a user can access your site you can configure the system to restrict access by IP Address. To set this up navigate to Settings -> IP Restrictions.
When you get to that page you will see the IP address where you are accessing the application from along with a button to configuration the IP addresses.
Click the edit icon. You will be able to enable the restriction and enter the allowed IP addresses.
There is also a setting on whether users with 'System Admin' permission are allowed to bypass that check. In other words - users with that elevated privilege can access the site from anywhere.