AWS Certified Advanced Networking Specialty Practice Exam

Attaching a network ACL (Access Control List) to a subnet indeed means that the network ACL acts as a firewall for controlling traffic at the subnet level. This is a fundamental feature of network ACLs within AWS, as they enable you to define both inbound and outbound rules that govern traffic flow to and from resources in a specific subnet.

Network ACLs operate at the layer of IP addresses, providing a means to allow or deny traffic based on defined rules. Each rule can specify protocols, ports, and source or destination IPs. This granular control is essential for securing the network while ensuring that necessary communications can occur.

By applying a network ACL to a subnet, you can enhance security posture, enforce compliance, and manage access levels to the resources within that subnet effectively. This functionality is particularly critical in a shared environment, where various applications may run on different resources in the same VPC.

The other options do not accurately represent the primary function of a network ACL. It does not provide additional storage, increase bandwidth, or function as a backup mechanism for data in the subnet. Instead, its core purpose is to filter traffic, which is why the first choice is the correct understanding of the result of attaching a network ACL to a subnet.