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Difficulty: EasyFirewalls and Access Control Lists (ACLs)

A network administrator configures a standard router Access Control List (ACL) to permit web traffic to an internal server. After applying the rule, the administrator observes that all other inbound network traffic to that interface is blocked, even though no specific deny statements were explicitly written. Which of the following explains this behavior?

  1. ACLs conclude with an unwritten implicit deny rule that drops all traffic not explicitly permitted.Answer
  2. B
    Standard ACLs operate at Layer 7 of the OSI model and automatically drop unencrypted application payloads.
  3. C
    The router interface automatically translates all unpermitted IP packets into UDP datagrams before discarding them.
  4. D
    Permitting web traffic on port 80 automatically forces the router to perform PAT overload on all non-HTTP packets.

Answer

Access Control Lists end with an unwritten implicit deny statement that drops any packet not explicitly allowed by a preceding rule.
Every Access Control List contains an invisible, unwritten implicit deny all rule at the end of the rule list. If incoming traffic does not match any explicit permit statement, it hits this implicit deny entry and is dropped by the router.

Step-by-Step Solution

1
Analyze the ACL evaluation process for incoming network packets.
The router checks packet fields top-down against each sequential rule in the list.
ACL entries are evaluated in sequential order until a match is found.
2
Determine packet disposition when no configured rules match the traffic.
The packet hits the invisible final statement at the bottom of the ACL list.
All standard and extended ACLs implicitly terminate with a deny-all statement to ensure a secure default-deny posture.

Key Concept

ACL Implicit Deny Behavior
Estimated Time:45s
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Topics

Firewalls and Access Control Lists (ACLs)

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