No, you are not obliged to send over the user IP Addresses in your requests to get access to our Car Hire APIs.
However, please note that a proportion of our Car Hire Supply Partners require you to send the IP Address information of your users – about 50% of all quotes returned via the Car Hire API come from these Supply Partners. There are many reasons for this requirement; most of them have to do with user authenticity checks, accurate geo-pricing, as well as providing correct offers for things such as insurance (more details here).
All in all, these are Supplier policies and Skyscanner can only provide access to these Suppliers if IP Addresses are passed. Please let us know if you require more details regarding the exact list of the Supply Partners involved, as well as the markets in which they are most prevalent. Depending on your decision, we can offer you more details regarding next steps in this regard.