What is the primary function of an Access Control List (ACL)?

The primary function of an Access Control List (ACL) is to control the flow of traffic in a network. ACLs are used to define rules that determine which packets are allowed to enter or exit a network interface based on criteria such as IP address, protocol, and port numbers. By configuring these rules, network administrators can effectively manage access to network resources, enhance security, and implement policies that restrict unauthorized access to sensitive information.

ACLs operate at various layers of the OSI model, primarily at the Network Layer (Layer 3) and Transport Layer (Layer 4), making them essential tools for firewalls and routers to filter traffic based on predefined criteria. This traffic control aspect helps to ensure that only legitimate users and services have access to specific resources while preventing unwanted traffic from compromising the network's integrity.

Options related to encrypting data, tracking user activities, and managing physical security do not align with the primary role of ACLs, which focus specifically on network traffic management and access control, thereby reinforcing the correct answer choice.