Q:

Compute Engine Firewall for App Engine Url Fetch

I've got an app running in Google App Engine that accesses a service run off a machine in Google Compute Engine. The Google App Engine application is the only accepted client for this service.
Is there a way for me to add a firewall rule to give the Google App Engine, specifically the Url Fetch Service, access to Compute Engine?
I don't want to give the whole world access to this server, but it appears that the Url Fetch Utility doesn't publish the list of outgoing IP addresses it uses.

A:

URL Fetch uses IP addresses specified here:
_cloud-netblocks.googleusercontent.com
Of course those IP addresses are shared by multiple Applicationss, and are also used by other services than URL Fetch.  Thus, you should not reply on firewalling as your primary means of authentication.

