Subnets

WHAT IS IT? [Azure Virtual Network/ Google VPC Virtual Network]
Shorthand for subnetwork, a subnet is just a subsection of a network. When you create a VPC, you’ll have a series of subnets associated with all the applications within a specific availability zone. We’ll provision resources, like an EC2 instance or an RDS database, within particular subnets—which can be public or private. Public subnets have a route to the internet that’s associated with an internet gateway. Public subnets can also talk to other public subnets. Private subnets do not have a path to the internet, but they can connect to public subnets within your VPC.

WHAT’S THE FUSS?

You can’t have one subnet across multiple availability zones. You’ll probably hear something along the lines of “one subnet equals one availability zone.” Let’s say you’ve decided to launch a VPC within a particular region, and within that region, AWS offers a set of availability zones. If you’d like to keep some information private—such as a set of customer information in an RDS database—you would launch a private subnet within one availability zone.
However, suppose you wanted to launch a subnet within a different availability zone (such as looking for some redundancy). In that case, that subnet will not be able to talk to your private subnet in a different availability zone. Your private subnet won’t span multiple availability zones. So this makes it an important consideration when figuring out how to handle disaster recovery.