What is an optional security control that can be applied at the subnet layer of a VPC?

A Network ACL (Access Control List) serves as an optional security control that can be implemented at the subnet level within a Virtual Private Cloud (VPC) on AWS. Network ACLs function as a stateless layer of security, meaning they evaluate each individual request to allow or deny traffic based solely on the rules defined within the ACL. This is particularly useful for controlling incoming and outgoing traffic to and from subnets, thereby adding an additional degree of security to your networking configuration.

Network ACLs can be tailored to apply broad rules that govern entire subnets, allowing for simpler management of access controls in settings where multiple resources may share the same subnet. Administrators can define rules based on various criteria, including IP protocols, port numbers, and source and destination IP addresses.

In contrast to Network ACLs, security groups operate at the instance level and are stateful, meaning they automatically allow return traffic for established connections, making them suitable for individual instances rather than entire subnets. IAM Policies are used for managing access permissions for AWS service resources rather than network traffic, and while firewall rules might conceptually operate similarly, they are not an AWS-native feature specifically associated with VPCs. Thus, the Network ACL is the correct and relevant choice in the context