An AWS subnet is a set of IP addresses, routing rules, and network access control lists (ACLs) that can be applied to a collection of virtual machines. The subnet groups the virtual machines together and controls their access to anything outside of the group.
Notably, an AWS subnet lives in a single availability zone (AZ). Therefore in all of our VPCs there will be two or more subnets for each purpose, each in a different AZ.
In the standard OptimalBI pattern we have: