ACLs

ACLs, or Access Control Lists, are ordered sets of rules applied to a router or switch interface that permit or deny traffic based on criteria such as source IP address, destination IP address, protocol, and port number. They are one of the most fundamental tools for controlling traffic flow and enforcing security policies on a network. ACLs can be applied inbound or outbound on an interface and are evaluated sequentially, with the first matching rule determining the action taken on the packet.