You will create a DB subnet group that is a collection of subnets (private subnets) designated for your DB instance.
3.2.1. Open the Amazon RDS console at https://console.aws.amazon.com/rds.
3.2.2. In the navigation pane, choose Subnet groups and click on Create DB Subnet Group.
3.2.3. For Name and Description type PrivateDBGroup
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3.2.4. For the VPC select My VPC.
3.2.5. In Add subnets section, select the Private Subnets, from us-east-1a select 10.0.2.0/24 (Private Subnet 01), and for us-east-1b select 10.0.3.0/24 (Private Subnet 02). Choose Create.