All instaces in the same VPC network are able to communicate to each other by using internal IPs of each instance's port. In case that communication across VPC networks are required, there are 2 approaches available as follows:
1. Through internet: The process of NAT (Network Address Translation) will translate the internal IPs into external IPs allowing your instance to communicate with an external network. NIPA Cloud Space uses the fabric network technology from Tungsten Fabric to function this method. (On the diagram, please see the arrow (1.) directing from instance-1 to 4 through the external network)
2. Through internal network: each instance can communicate across projects if the VPC network of each port has an interface that connects to a router. This medthod is free to use. (On the diagram, please see the arrow (2.) directing from instance-1 to 4 through the router)
Every instance's communication needs to operate through ports. For more security, you can install the security group at the ports in order to function as a firewall, blocking out unwanted traffics to keep your instance safer.