AWS VPC 101: What is a VPC, topology, VPC access, and packet flow

How would the packets flow to instances B and C?

Remember, instance B is living in two subnets (Subnet 1 and 2), and instance C which is living in subnet 3.

If instance B wanted to talk to instance C, it can go one of two ways. It could go out of subnet 1 or subnet 2. Regardless of which subnet it goes out on, the same rules apply. It’s going to hit the routing table, go to the firewall, source destination check, and security group out.

It’s going to check the route table to make sure it has a route to that destination network, and then because it’s going to a different network, it’s going to check the network ACL out, and then on the reverse side, it comes back in. It’s going to check the network ACL in before it checks the security group, so it’s different types of connectivity options for instances that happen to live in a different subnet.

Understanding the above flow can be very useful. Once you understand how the packets flow and where they were going, and how everything was being checked, it allows you to better troubleshoot VPC network connectivity issues.

Once you grasp the VPC networking concepts in this blog, we suggest you have a look at our “AWS VPC Best Practices” blog post. In it, we share a detailed look at best practices for the configuration of an AWS VPC and common VPC configuration errors.