Hi, The URL Fetch service is not available for Node.js on App Engine Flexible. However, for cross-service communication, you can send HTTP requests to a service by including its name in the URL or use Cloud Pub/Sub. You may use the Google Cloud Client library for Node.js to access the Pub/Sub service from a Node.js app.
Access to a Flex app is through the HTTP(S) load balancer which currently does not have the ability to access to IP addresses (since it doesn’t provide static IPs). I should add that HTTPS load balancer for GAE Flex provides a public IP address. When you route to that IP address from a Flex instance, you are going out of the GCE virtual network and then back in again.