Understanding routes and firewalls in Google Cloud
By default, every network in GCP will have routes automatically created to handle reachability between instances in the network (regardless of whether they are on the same subnetwork), and also a default route for traffic leaving the network. The actual network routers along the way are not something you manage or even see. They're entirely abstracted away and treated as a single centralized virtual router, which every instance connects to. Through Google's Cloud Routes service, you can create your own custom routes by defining a next hop IP address for traffic destined to any network you specify (via its IP address range). You can also apply tags to routes so that they only apply to specific instances with the corresponding tag, but otherwise, routes apply to all the instances in the network by default.
The Firewall service in GCP functions as a distributed stateful firewall across the VPC network. This means that firewall...