Which statement about using an Access Control List on a router is true?

• A. ACLs automatically optimize routing tables.
• B. ACLs are used to filter traffic and enforce security policies between networks.
• C. ACLs have no effect on inter-network security.
• D. ACLs replace the need for firewalls.

Answer: (B)

ACLs on a router are used to filter traffic and enforce security policies between networks. They inspect packets as they cross an interface and permit or deny them based on rules you define, such as source and destination IPs, protocols, and ports. This lets you block unauthorized traffic, restrict access to certain resources, and shape what can travel between different parts of the network, effectively enforcing security boundaries across networks. ACLs operate at Layer 3 (and can use Layer 4 details in extended forms) and are evaluated in order, with the first matching rule deciding the outcome. They can be placed inbound or outbound on interfaces and come in standard (source-based) or extended (more granular) varieties. They don’t optimize routing tables, they aren’t a replacement for firewalls, and they do have a direct impact on inter-network security.