How to use the classic link?
To use a classic link, the users have to enable minimum one virtual private cloud on their account for the traditional connection. Later, they can associate a security group from that virtual private cloud to the EC2 classic instance that they would prefer.
This process will make sure that the EC2 classic instance is linked to VPC and will become s member of the selected security group of the VPC. The EC2 classic instant cannot be connected to more than one VPC at the same time.