Prismic is hosted on AWS servers. If you need to know the IP Ranges that Prismic uses, you can information about this here:
Allowing calls made from the Prismic app
If you want to allow calls made from the app itself (when editors are using the Writing Room), IP ranges are listed here.
The Prismic web app is running on us-east-1 region.
IP Ranges for assets
If you're looking for the IP ranges used for the API and assets that are delivered when using the CDN, you find information here about the Cloudfront IP list.