The answer is that the ICMP echo request is denied by the implicit deny. Every standard and extended ACL on a Cisco router ends with an invisible, automatic rule that denies all traffic not explicitly permitted by a preceding entry. Since the only permit statement in this ACL matches TCP traffic from the 10.10.10.0/24 network to host 192.0.2.10 on port 443 (HTTPS), an ICMP packet—which uses a different protocol and no port—fails to match any permit line and is therefore dropped by the implicit deny. On the CCNA 200-301 v2 exam, this concept tests your understanding that ACLs are not “permit all” by default; a common trap is forgetting that the implicit deny exists, leading students to mistakenly think unmatched traffic is allowed. Remember the mnemonic: “If it’s not explicitly permitted, it’s implicitly denied.”
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: aCLs process packets sequentially and apply the first matching permit or deny rule to the traffic.. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An ACL permits only tcp 10.10.10.0/24 host 192.0.2.10 eq 443 and has no other permit entries. What happens to an ICMP echo request from 10.10.10.5 to 192.0.2.10?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It is denied by the implicit deny
ACLs end with an implicit deny. Since the only explicit permit is for HTTPS traffic, the ICMP packet is denied.
Key principle: ACLs process packets sequentially and apply the first matching permit or deny rule to the traffic.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
It is permitted because the destination matches
Why it's wrong here
The destination match alone is not enough when protocol and port do not match.
When this WOULD be correct
In a different exam scenario where an ACL permits ICMP traffic from a specific source to a specific destination, such as 'permit icmp 10.10.10.0/24 host 192.0.2.10', this option would be correct as the ICMP echo request would match the permit statement.
✓
It is denied by the implicit deny
Why this is correct
Correct. Traffic not explicitly permitted is denied.
Related concept
ACLs process packets sequentially and apply the first matching permit or deny rule to the traffic.
✗
It is permitted because the packet matches the source network stated in the ACE.
In a different scenario where an ACL permits both TCP and ICMP traffic, a question might ask about the behavior of an ICMP packet under those conditions. If the ACL included rules for ICMP, then translating ICMP to TCP could be a valid consideration.
In a different scenario, if the ACL specified a rule that allowed ICMP traffic from any source to a specific destination, and the question asked about ICMP traffic, then this option could be correct if the source port was relevant to the rule being applied.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓It is denied by the implicit denyCorrect answer▾
Why this is correct
Correct. Traffic not explicitly permitted is denied.
✗It is permitted because the destination matchesWrong answer — click to see why▾
Why this is wrong here
The ACL only permits TCP traffic from 10.10.10.0/24 to host 192.0.2.10 on port 443. ICMP is not TCP, so even though the destination matches, the protocol does not match, and the packet is not permitted.
★ When this WOULD be the correct answer
In a different exam scenario where an ACL permits ICMP traffic from a specific source to a specific destination, such as 'permit icmp 10.10.10.0/24 host 192.0.2.10', this option would be correct as the ICMP echo request would match the permit statement.
Why candidates choose this
Students might think that matching the destination IP address is sufficient for permit, but ACLs require matching all specified fields (protocol, source, destination, and port).
✗It is permitted because the packet matches the source network stated in the ACE.Wrong answer — click to see why▾
Why this is wrong here
Although the source IP matches the ACE's source network, the ACE only permits TCP traffic; ICMP is a different protocol, so the packet is denied by the implicit deny.
★ When this WOULD be the correct answer
In a different scenario where an ACL permits both TCP and ICMP traffic, a question might ask about the behavior of an ICMP packet under those conditions. If the ACL included rules for ICMP, then translating ICMP to TCP could be a valid consideration.
Why candidates choose this
Some students may confuse ACLs with NAT or other features that can modify packets, but ACLs only filter based on existing packet headers.
✗It is permitted only if the source port is 443Wrong answer — click to see why▾
Why this is wrong here
ICMP does not use TCP ports; it uses ICMP type and code. The ACL entry specifies TCP port 443, which is irrelevant for ICMP traffic. Even if the source port were 443, ICMP packets do not have TCP ports.
★ When this WOULD be the correct answer
In a different scenario, if the ACL specified a rule that allowed ICMP traffic from any source to a specific destination, and the question asked about ICMP traffic, then this option could be correct if the source port was relevant to the rule being applied.
Why candidates choose this
Students might think that matching the source port 443 could permit the traffic, but they overlook that the protocol must also match TCP.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Common exam traps
Common exam trap: answer the scenario, not the keyword
Remember that ACLs have an implicit deny all rule. Just because a source or destination is specified doesn't mean all traffic types are allowed.
Detailed technical explanation
How to think about this question
Access Control Lists (ACLs) are fundamental security tools in Cisco networking that filter traffic based on defined criteria such as protocol type, source and destination IP addresses, and port numbers. ACLs process packets sequentially, matching each packet against the list entries until a permit or deny condition is met. If no explicit match is found, an implicit deny rule at the end of every ACL blocks the traffic by default. This implicit deny ensures that only traffic explicitly permitted by the ACL is allowed through the interface.
In the context of this question, the ACL explicitly permits only TCP traffic from the 10.10.10.0/24 subnet to the host 192.0.2.10 on destination port 443 (HTTPS). Since ICMP echo requests do not use TCP or port 443, they do not match the permit condition. Consequently, the implicit deny at the end of the ACL blocks the ICMP packet. Cisco routers do not translate ICMP packets into TCP, so the ICMP echo request is not converted or permitted under the existing ACL rules.
A common exam trap is assuming that matching the destination IP address alone is sufficient for permitting traffic. Candidates may incorrectly believe that because the destination host matches, the ICMP echo request should be allowed. However, ACLs evaluate all specified criteria, including protocol and port numbers, and only permit traffic that fully matches an explicit permit statement. Practically, this behavior enforces strict traffic filtering, ensuring that only authorized protocols and ports are accessible, which is critical for network security and compliance.
KKey Concepts to Remember
ACLs process packets sequentially and apply the first matching permit or deny rule to the traffic.
Every ACL ends with an implicit deny that blocks all traffic not explicitly permitted by earlier entries.
ACL permit statements must match all specified criteria, including protocol type, source and destination IPs, and port numbers.
ICMP traffic does not use TCP ports and cannot match ACL entries that specify TCP port numbers.
Cisco routers do not translate ICMP packets into TCP to bypass ACL restrictions.
An ACL permitting only TCP traffic on port 443 will deny all other protocols, including ICMP.
Implicit deny behavior enforces strict security by blocking any traffic not explicitly allowed.
Matching only the destination IP address in an ACL is insufficient to permit traffic if other criteria differ.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
ACLs process packets sequentially and apply the first matching permit or deny rule to the traffic.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review aCLs process packets sequentially and apply the first matching permit or deny rule to the traffic., then practise related 200-301 questions on the same topic to reinforce the concept.
Network Services and Security — This question tests Network Services and Security — ACLs process packets sequentially and apply the first matching permit or deny rule to the traffic..
What is the correct answer to this question?
The correct answer is: It is denied by the implicit deny — ACLs end with an implicit deny. Since the only explicit permit is for HTTPS traffic, the ICMP packet is denied.
What should I do if I get this 200-301 question wrong?
Review aCLs process packets sequentially and apply the first matching permit or deny rule to the traffic., then practise related 200-301 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
ACLs process packets sequentially and apply the first matching permit or deny rule to the traffic.
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