When configuring an ACL, if there is no specific rule for a certain type of traffic, what happens?

When configuring an Access Control List (ACL), if there is no specific rule that matches a certain type of traffic, that traffic is dropped. This behavior is due to the implicit "deny all" rule that is applied at the end of the ACL. Essentially, if the traffic does not meet any of the conditions specified in the ACL rules, it defaults to being denied, meaning it will not be allowed to pass through the interface where the ACL is applied.

This approach to traffic management ensures that only explicitly permitted types of traffic can traverse the network segment controlled by the ACL, enhancing security by preventing any undefined or potentially harmful traffic. ACLs operate on a first-match principle, meaning that the device checks the conditions in the order they are listed and applies the rule associated with the first match it finds. If no matches occur, the implicit deny action will take effect.