What should be used to define a list of rules for allowing or denying traffic in a subnet?

Network ACLs, or Network Access Control Lists, are used to define a list of rules that allow or deny traffic at the subnet level in Amazon Web Services (AWS). This provides a layer of security by controlling the inbound and outbound traffic to and from the resources within that subnet. Each rule consists of criteria like allowed protocols, ports, and IP address ranges, and it operates at the network layer, allowing for a more granular level of control.

Network ACLs are stateless, meaning that if an inbound request is allowed, the corresponding outbound reply is not automatically allowed; you must explicitly allow it in the ACL rules. This characteristic makes them suitable for firewall-like behavior on a subnet level.

In contrast, security groups operate at the instance level, providing rules that control traffic based on protocols, ports, and IP addresses specifically for EC2 instances. Routing tables are used to determine where network traffic is directed within the AWS environment, while Elastic Load Balancers distribute incoming application or network traffic across multiple targets, such as EC2 instances. Each of these serves different purposes and does not specifically define rules for traffic at the subnet level like Network ACLs do.