An ACL is essentially a packet filter.

Comprehensive Explanation: An ACL (Access Control List) is used to define rules for permitting or denying packets based on criteria such as IP addresses, protocols, or ports. However, the ACL only takes effect when it is applied to a service module, such as an interface or a firewall rule. This ensures that traffic is filtered according to the ACL rules as it passes through the device. Without being applied, the ACL rules are inactive and do not influence traffic.