Automatic login based on IP address

I’m looking for a related solution. I need to automatically log in users to specific accounts based on their IP address. Might this be possible? We use this kind of functionality to allow access to our site automatically from institutional sites like libraries. Ideally we’d like to be able to automatically login up to a specified number of users automatically (i.e. user comes from IP X, we check to see how many slots are available for that user (let’s say 5) and if there’s a slot, we log them into user account Y automatically, and they have access to whatever that user account has access to.