With GCP VPC peering, Atlas is assigned a large CIDR block which is then further subdivided as GCP resources are deployed across different regions. At this time that allocation is not visible to users, and cannot be directed or known before resources are deployed.

This makes it near impossible to apply any firewall rules, and requires you to allow the entire CIDR block instead of limiting it on a per region basis. We would like to be able to determine the subnet that is assigned to a region before resources are deployed, and to be able to see the CIDR blocks assigned to regions through the API or Atlas CLI to have further visibility.