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Difficulty: EasyFirewalls and Access Control Lists (ACLs)

A network technician applies an Access Control List (ACL) to a router interface containing a single entry that permits outbound HTTP traffic from an internal subnet. Immediately after applying the rule, users report that all non-HTTP network traffic originating from that subnet is blocked. Which fundamental ACL rule mechanism causes traffic not explicitly allowed to be dropped automatically?

  1. The implicit deny statement located at the end of the ACLAnswer
  2. B
    An automatic OSI layer mapping failure between Layer 2 frames and Layer 3 packets
  3. C
    A default protocol conversion error between TCP and UDP transport headers
  4. D
    The Port Address Translation (PAT) pool running out of available socket ports

Answer

The implicit deny statement located at the end of the ACL is responsible for dropping all traffic that is not explicitly permitted.
Access Control Lists operate with a top-down evaluation order ending with an invisible, automatic 'deny all' statement (implicit deny). When an ACL contains only one explicit permit rule (such as allowing HTTP traffic), any traffic that does not match that specific rule falls through to the end and is blocked by default.

Step-by-Step Solution

1
Analyze how Access Control Lists evaluate traffic against configured rules.
ACLs process packets top-down sequentially until a match is found.
If a packet matches a rule's criteria (such as source IP, destination IP, or port), the specified action (permit or deny) is executed immediately.
2
Identify default behavior when no explicit rules match the packet.
The packet falls through to the hidden final entry of the ACL.
All standard and extended ACLs contain an unwritten implicit deny rule (e.g., 'deny ip any any') at the very end of the list.
3
Determine the impact of having only a single permit rule.
Only traffic matching that specific permit rule passes; all other traffic is dropped by the implicit deny.
Because only HTTP traffic matched the sole permit statement, all other protocols hit the implicit deny and were blocked.

Key Concept

ACL Implicit Deny Rule
Estimated Time:45s
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Topics

Firewalls and Access Control Lists (ACLs)

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