An Access Control List (ACL) is a fundamental security mechanism in cybersecurity that defines which users, groups, or network entities are permitted to access specific resources and what actions they can perform. ACLs function as digital gatekeepers, containing ordered sets of rules that systematically evaluate incoming requests against predefined criteria to grant or deny access. These rules can apply to various system objects including files, directories, applications, network ports, and communication protocols.

ACLs are extensively deployed on network devices such as routers, switches, and firewalls to manage traffic flow, filter packets based on source and destination addresses, and restrict connectivity to sensitive infrastructure components. By enabling granular permission specifications, ACLs help organizations implement the principle of least privilege, minimizing attack surfaces and protecting the confidentiality, integrity, and availability of critical digital assets. Proper ACL configuration is essential for maintaining a secure and resilient operational environment.