Communication between instances within different projects

[Don Annas]

Hello, I wanted to confirm that traffic between instances within different projects reach each other via the public IP and are not routed privately or exempt of the NAT translation?  Is this the case?  Thank you. 

[Carlos (Cloud Platform Support)]

Hi Don, 

You are right, communication between instances in two different projects will take place via external IPs.  Furthermore, as stated in this document: ¨Any communication between instances in different networks, even within the same project, must be through external IP addresses.¨

If you need the VMs in different networks (which apply to VMs in two different projects) to communicate via private IPs you can always setup a Cloud VPN. 

Let me know if this helps.