Hello, everyone!
@jlj7 is correct that the IPs for Terraform Cloud are obtained with the API from this page.
Note that those IP ranges are for Version Control Systems, Sentinel, and Notifications only. HashiCorp Terraform Cloud Business Tier allows for agents that solve the remainder of this scenario by providing the ability to use Terraform Cloud Agents which allow Terraform Cloud to communicate with isolated, private, or on-premises infrastructure.
Please let us know if that answers your questions, and if you have any other concerns!