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Difficulty: EasyFirewalls and Access Control Lists (ACLs)

After applying a newly defined Access Control List (ACL) containing only a single permit statement for a specific management host onto a router interface, a network technician notices that all traffic from other hosts on the network is immediately blocked. What default ACL behavior causes this traffic to be dropped?

  1. An unwritten implicit deny rule at the end of the ACL automatically drops any traffic that does not match an explicit permit rule.Answer
  2. B
    ACLs process rules strictly at OSI Layer 2, causing all Layer 3 IP traffic to fail evaluation.
  3. C
    The router automatically enables Port Address Translation (PAT) overload, suppressing all unmapped private source IP addresses.
  4. D
    ACLs require all connectionless UDP protocols to be manually encapsulated into TCP before evaluation.

Answer

An unwritten implicit deny rule at the end of the ACL automatically drops any traffic that does not match an explicit permit rule.
All standard and extended ACLs feature a invisible 'deny all' statement at the very end of the list. If a packet does not match any explicit permit statements defined by the administrator, it hits this final implicit deny rule and is dropped.

Step-by-Step Solution

1
Analyze how router ACLs evaluate incoming or outgoing packets against configured rules.
ACLs inspect packets top-down in sequential order until a matching rule is found.
Sequential processing ensures that specific traffic controls take precedence based on line number.
2
Determine what happens when a packet reaches the end of an ACL without matching any explicit rule.
The packet encounters the default 'deny ip any any' rule hidden at the end of every ACL.
Network security standards mandate a default-deny posture to ensure unapproved traffic cannot bypass security filtering.

Key Concept

ACL Implicit Deny Rule
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Topics

Firewalls and Access Control Lists (ACLs)

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