How to avoid direct access to default page for normal users in JPP 6 ?

Issue

• We have our portal published into internet. There are two types of users on our portal: normal user and administrator users.
  Normal users login into portal by using another application, and using SSO they enter into our portal.
  Also we have administrator users. They should be able to login into portal by using normal/default login page (from portal).
  For security reason, we want to avoid that any user not included in a range of IPs should be able to try login page.
  Are there any way to implement this kind of security restriction?