Resources that are in the same VPC can communicate using internal IP addresses, as well as using a project-internal DNS facility. This is true even if the resources are in different regions. For instance, consider two VMs, one in the US and the other in the UK; provided these are in the same VPC, they will be able to communicate using internal IP addresses despite their physical distance.
By contrast, if two resources are in different VPCs, even if they happen to be in the same region or even on the same underlying bare metal box (remember that GCP VMs are multi-tenanted), they will still have to communicate using external IP addresses, which implies that the network traffic between them will have to pass over the internet (or Google’s global network in this case).
Internal IP addresses are assigned from the subnet range using the familiar...
