It is not possible to whitelist the IP range for our REST API since our IP addresses are dynamic. Our REST API platform is offered from a globally distributed infrastructure. Hence you will not be able to whitelist the IP-addresses of our platform.
We do offer, however, the possibility of using a signing key to verify HTTP requests. You can create a signing key here in the Dashboard, and there is more information on how to implement it into your API set up here in the Documentation.