CompTIA Network+ Practice Test

What happens when an access list with no rules is applied to an interface?

1. All traffic is allowed

2. Traffic is filtered according to priority

3. All traffic is filtered out

4. Only administrative traffic is allowed

The correct answer is: All traffic is filtered out

When an access list with no rules is applied to an interface, it functions as an implicit deny all policy. This means that any traffic that does not explicitly match a permit rule is denied by default. Since there are no rules to allow any specific traffic, all traffic is effectively filtered out. Implicit deny is a fundamental security principle used in firewalls and access control lists: traffic that is not explicitly permitted is automatically denied. Therefore, when no rules are defined, it is treated as if there is a blanket rule that denies everything. This ensures that only traffic that is purposely allowed via additional rules would be permitted, which enhances security by preventing any unwanted access.