In what order do routers evaluate the Access Control Entries within an ACL?

Routers evaluate the Access Control Entries (ACEs) in an Access Control List (ACL) in a top-to-bottom manner. This means that the router will start at the first entry in the list and continue to evaluate each subsequent entry until it finds a match for a packet. Once a match is found, the action defined in that ACE (permit or deny) is applied, and no further entries are assessed.

This sequential evaluation is critical in determining how traffic is allowed or blocked, as the order of the entries significantly impacts the overall behavior of the ACL. If a specific condition is placed higher in the list, it will take precedence over a more general condition placed lower down.

For example, if there is a specific rule to deny a certain IP address located at the top, and a more general rule to permit traffic located further down, any packets matching the denied IP will be blocked even if they would subsequently be permitted by the general rule. This top-to-bottom evaluation ensures that network administrators must carefully plan the order of entries in their ACLs to achieve the desired security policies effectively.