When cloud resources in a VPC need to communicate with the Internet, you can use products such as NAT Gateway, EIP, and Server Load Balancer to connect the cloud resources to the Internet.
For more information, see How to choose Internet-facing products in VPC.
When cloud resources in a VPC need to communicate with resources in another VPC or an on-premises IDC, you can use VPN Gateway or Express Connect to build secure and reliable private network connection. You can also use both products to expand the network topology and build a hybrid cloud.
When the ECS instances in the classic network need to communicate with the cloud resources in a VPC, enable the ClassicLink function of the VPC. After the ClassicLink connection is established, ECS instances in the VPC can access the ECS instance in the classic network, and the ECS instance in the classic network can access all cloud resources such as ECS instances, SLB instances and RDS in the VPC.
For more information, see ClassicLink overview.