What is an ACL?

• A. Automatic Link Control
• B. Access Control List filtering traffic based on IP, port, protocol; can block seemingly "healthy" flows.
• C. A protocol for linking devices
• D. A method to encrypt traffic

Answer: (B)

An ACL is a set of rules applied by a router or switch to filter traffic. Each rule specifies criteria like the source and destination IP addresses, port numbers, and protocol, and the device enforces the rule by permitting or denying packets that match. Rules are evaluated in order, and the first match wins. If no rule matches, traffic is typically dropped by default (implicit deny). This lets you block traffic that would otherwise be allowed if it matches a deny rule, or permit only specific traffic, aligning with security and traffic-management goals. It’s not Automatic Link Control, not a protocol for linking devices, and not a method to encrypt traffic.