To enforce private connectivity between on-premises hosts and Google Cloud APIs while optimizing for cost and operational efficiency, using a dedicated or Partner Interconnect is the best solution. This setup ensures a reliable, high-bandwidth connection with private IP addressing.

Choose Interconnect Type: Decide between Dedicated Interconnect and Partner Interconnect based on your bandwidth needs and proximity to Google Cloud locations.

Set Up Interconnect:

For Dedicated Interconnect, order circuits through the Google Cloud Console.

For Partner Interconnect, select a supported service provider and order the connection through them.

Configure VPC and Private Google Access:

In your VPC, enable Private Google Access to allow on-premises hosts to access Google APIs privately.

Go to "VPC network" -> "Private Google Access" and enable it for your subnets.

Establish Connectivity: Work with your network team and (if applicable) your Partner Interconnect provider to set up the physical and logical connections.

Test Connectivity: Verify that on-premises hosts can reach Google Cloud services using private IP addresses.