ACL (Access Control List) refers primarily to two technical concepts.
The first is a file-system ACL, specifying user or group permissions to access files, directories, or system resources. Operating systems typically reference these ACLs—configured by file owners or system administrators—to determine whether operations such as read, write, or execute are permitted for a given user or group.
The second is a network ACL, typically configured on devices such as routers and firewalls, defining policies to permit or deny network packet flows. Network ACLs evaluate packets based on various parameters including source IP address, destination IP address, source port number, and destination port number. This evaluation is sequential, starting from the top of the ruleset, and the first rule matched dictates the action (allow or deny) applied to the packet. Subsequent rules are ignored after the initial match. If a packet does not match any explicit ACL rules, an "implicit deny" action is applied by default, rejecting the packet.
Additionally, devices like F5 BIG-IP can implement advanced, session-aware ACLs applied dynamically at Layers 4–7 (e.g., HTTP application-layer controls). These advanced ACL capabilities enable finely-grained access policies. For example, internal corporate users can be granted full access to internal business applications, while guest or external users can be restricted only to publicly accessible web resources.