Final answer:
An ACL (Access Control List) is used by network devices to verify permissions for data packets, allowing or denying them based on predefined rules about IP addresses, ports, and protocols.
Step-by-step explanation:
An ACL, or Access Control List, is a table that a network device like a router or switch uses to verify the permissions of incoming and outgoing packets. The ACL checks the data packets against a set of rules that determine whether the packets are allowed or denied to pass through the network device. ACLs are used to provide a basic level of security for network traffic, and they can be configured to filter traffic based on criteria such as source and destination IP addresses, port numbers, and protocols.
For example, an ACL could be set up to block all incoming traffic from a particular IP address or to permit only HTTP and HTTPS traffic to access a web server. Furthermore, ACLs can be used to define the types of traffic that are allowed to exit a network, thereby preventing sensitive data from being transmitted outside of a secure network environment.