RFI: certain IP’s must use password

The site is open to the world–but I want to restrict users from certain IP ranges to have to login…(only a few will be granted while the majority of them get a 404 or whatever). Everyone else in the world has open access.

background: just to add this to ward off misdirected comments…the people in this IP range cannot physically use any other IP ranges….ever. So controlling their access thru IP means will be absloute on this topic.

Any ideas? All the other passwording questions I’ve found are for something kinda opposite of this.