Can Subnets Communicate with Each Other?

Different VPCs cannot communicate with each other, so subnets in different VPCs are isolated from each other.

Subnets in the same VPC can communicate with each other by default. If network ACLs and security groups are used to protect network security, communications between subnets may be denied by these rules.

Network ACL: If you associate subnets with different network ACLs and do not add inbound and outbound allow rules, communications between these subnets would fail.
Security group: If you associate instances (such as ECSs) in a subnet with different security groups and do not add inbound and outbound allow rules, communications between these instances would fail.

If both network ACLs and security groups are configured, traffic preferentially matches the network ACL rules. For details, see Table 1.

Figure 1 Communications between subnets in a VPC

**Table 1** Communication scenarios
Scenario	Access Control Configuration	Description
Between subnets	No network ACLs associated Instances associated with the same security group	Subnets 3 and 4 are not associated with a network ACL, so they can communicate with each other. ECS07, ECS08, ECS09, and ECS10 are associated with the same security group (security group A), so they can communicate with each other.
	Subnet associated with the same network ACL Instances associated with different security groups	Subnets 1 and 2 are associated with the same network ACL (network ACL A), so they can communicate with each other. ECS01 and ECS02 in subnet 1 are associated with security group A, and ECS05 and ECS06 in subnet 2 are associated with security group B. If security groups A and B have no allow rules, ECSs in the two security groups cannot communicate with each other. For example, ECS01 and ECS05 cannot communicate with each other.
	Subnet associated with different network ACLs	Subnet 1 is associated with network ACL A, and subnet 5 is associated with network ACL B. If network ACLs A and B have no allow rules, subnet 1 and subnet 5 cannot communicate with each other. As a result, ECSs in subnets 1 and 5 are blocked from each other even they are in the same security group. For example, ECS01 and ECS 11 cannot communicate with each other.
Within a subnet	Instances associated with different security groups	ECS01 and ECS02 in subnet 1 are associated with security group A, and ECS03 and ECS04 are associated with security group B. If security groups A and B have no allow rules, ECSs in the two security groups cannot communicate with each other even they are in the same subnet (subnet 1). For example, ECS01 and ECS03 cannot communicate with each other.