Security groups and network ACLs are two AWS services that can be used to block network traffic to an instance. Security groups are virtual firewalls that control the inbound and outbound traffic for your instances at the instance level. You can specify which protocols, ports, and source or destination IP addresses are allowed or denied for each instance. Security groups are stateful, which means that they automatically allow return traffic for any allowed inbound or outbound traffic123. Network ACLs are virtual firewalls that control the inbound and outbound traffic for your subnets at the subnet level. You can create rules to allow or deny traffic based on protocols, ports, and source or destination IP addresses. Network ACLs are stateless, which means that you have to explicitly allow return traffic for any allowed inbound or outbound traffic456. References: 1: Security groups for your VPC - Amazon Virtual Private Cloud, 2: Security Groups for Your VPC - Amazon Elastic Compute Cloud, 3: AWS Security Groups: Everything You Need to Know, 4: Network ACLs - Amazon Virtual Private Cloud, 5: Control traffic to subnetsusing network ACLs - Amazon Virtual Private Cloud, 6: AWS Network ACLs: Everything You Need to Know