Formstack automatically collects the IP addresses of users submitting forms. You can find this info on the Submissions tab of your form by clicking on each individual submission.
You may to insert these IP addresses into the Notification or Confirmation emails. To do so, in the Form Field Panels, drag and drop the IP Address variable. It will be at the bottom of the list or you can use the search function. The variable will be replaced with the IP address of the person submitting your form.
You can also insert variable code, including the IP Address into the Subject Line of the Notification or Confirmation email by dragging and dropping it.