An inbound security group rule enables external access to ECSs in a security group, and an outbound security group rule enables ECSs in a security group to access external networks. If a security group has no access rules after an ECS is added to the security group, the communication between the ECS and the external network is blocked. The default inbound rule enables an ECS to be accessed by other ECSs in the same security group, and the default outbound rule enables ECSs in the security group to access external networks. Security groups cannot resolve the problems caused by network faults or incorrect network configuration. For example, when two ECSs cannot communicate with each other due to the network configuration, they still cannot communicate with each other even if you configure a security group rule to allow the communication between them.