I read an article by David Robinson: https://stackoverflow.blog/2017/02/15/how-do-students-use-stack-overflow
He filters questions posted by users at educational institutions identified by their ISP. I would like to do something like this for the company I work at.
Unfortunately, the appropriate query is not given. I tried to figure it out using http://api.stackexchange.com/docs/questions but cannot find the appropriate fields in the filter selection.
This is not possible for mere users, you need to be a Stack Exchange developer with direct access to the database to do so. IP addresses are personal information, and while they are under circumstances accessible to ♦ moderators this wouldn't be sufficient to do a query like you propose.
The API will never contain this information as it would be a major breach of trust (and possibly legally sanctionable, depending on the jurisdiction).
If you believe that you have a valid use for this data, you can contact the Stack Exchange team via the Contact Us form (linked in the footer) and explain your research objectives.
Be prepared to sign an NDA, at a minimum.
