What defines the typical default action of an ACL when no explicit allow rule is matched?

The typical default action of an Access Control List (ACL) when no explicit allow rule is matched is to deny all traffic. This means that if a packet does not match any of the defined rules in the ACL that permit certain traffic, it will be dropped by default. The rationale behind this design is to ensure security; by denying all unspecified traffic, network administrators can prevent unauthorized access and potential threats from entering the network.

It's important to understand that while there are additional behaviors in ACL implementations, such as logging or using other protocols, the foundational concept remains that the absence of an allow rule results in a denial. This default action ensures that any traffic not explicitly permitted is treated as disallowed, thus maintaining a restrictive posture unless specified otherwise through explicit rules.