Amazon Virtual Private Cloud (VPCs) and Amazon RDS
There are two Amazon Elastic Compute Cloud (EC2) platforms that host Amazon RDS DB instances, EC2-VPC and EC2-Classic. Amazon Virtual Private Cloud (Amazon VPC) lets you launch AWS resources, such as Amazon Relational Database Service (Amazon RDS) DB instances, into a virtual private cloud (VPC).
When you use an Amazon VPC, you have control over your virtual networking environment: you can select your own IP address range, create subnets, and configure routing and access control lists. The basic functionality of Amazon RDS is the same whether your DB instance is running in an Amazon VPC or not: Amazon RDS manages backups, software patching, automatic failure detection, and recovery. There is no additional cost to run your DB instance in Amazon VPC.
Accounts that support only the EC2-VPC platform have a default VPC. All new DB instances are created in the default VPC unless you specify otherwise. If you are a new Amazon RDS customer, if you have never created a DB instance before, or if you are creating a DB instance in a region you have not used before, you are most likely on the EC2-VPC platform and have a default VPC.
Some legacy DB instances on the EC2-Classic platform are not in a VPC. The legacy EC2-Classic platform does not have a default VPC, but as is true for either platform, you can create your own VPC and specify that a DB instance be located in that VPC.