Outbound services, such as the URL Fetch, Sockets, and Mail APIs, make use of a large pool of IP addresses. The IP address ranges in this pool are subject to routine changes. In fact, two sequential API calls from the same application may appear to originate from two different IP addresses.
The outbound addresses that App Engine uses falls within these IP address ranges.
Google publishes the complete list of IP ranges that it makes available to users on the internet in goog.json.
Google also publishes a list of global and regional external IP addresses ranges available for customer's Google Cloud resources in cloud.json.
The IP addresses used by Google APIs and services fit
within the list of ranges computed by taking away all ranges in
from those in