Let's look into routing within the GCP. As a reminder, this section does not cover the basics of network routing in general but talks about how routes are defined within the GCP environment. A route is a networking mapping of an IP range to its destination. Every network in GCP has routes in places that allow an instance to send traffic to each other or even across multiple subnets across different regions. Along with these routes, every network has a default route that directs traffic to outside the network. If you want to change the default route, you can do so by overriding it with a custom static route. If a route is in place, the only way to prevent an instance from talking to another instance is by using firewall rules. We will learn more about firewall rules in the next section.
When you create a VPC network, a default route for internet traffic (0/0) is created...