What is the primary function of an ACL in network security?

An Access Control List (ACL) plays a crucial role in network security by defining rules that determine which packets can enter or leave a network interface. Its primary function is to block unauthorized access, thereby protecting network resources from malicious users or unwanted traffic. By selectively permitting or denying traffic based on various criteria such as IP addresses, protocols, and ports, ACLs effectively enhance the security posture of a network. This control helps enforce a security policy, ensuring that only legitimate users and data are allowed through while preventing unauthorized access.

The other options represent different concepts in networking but do not align with the specific function of an ACL in security. Allowing all traffic would negate the purpose of controlling access and could lead to potential vulnerabilities. Managing bandwidth focuses on controlling the data throughput rather than access permissions, and establishing connections is more related to protocols like TCP or link-layer operations, which are not the primary concern of an ACL.