The docs say "You can use sockets to connect Google App Engine apps to
Google Compute Engine instances."
Is the limitation still that it can't communicate on the same network and
must go through the external IP of the compute engine instance?
Can app engine Felxible connect to the internal ip ad
Flexible runtime app instances exist within the same network resources as
GCE instances, because they are GCE instances. In that way, the GAE
Flexible instance can be configured to run in any existing network resource
and communicate to other instances using the local IP or hostname. For
Standard