This CC practice question tests your understanding of security principles. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. 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.
Exhibit
Refer to the exhibit.
```
interface GigabitEthernet0/1
ip access-group BLOCK_TELNET in
!
ip access-list extended BLOCK_TELNET
deny tcp any any eq 23
permit ip any any
```
A network administrator configures the ACL on a router as shown. What is the effect of this access list?
Refer to the exhibit.
```
interface GigabitEthernet0/1
ip access-group BLOCK_TELNET in
!
ip access-list extended BLOCK_TELNET
deny tcp any any eq 23
permit ip any any
```
A
Blocks Telnet traffic entering the interface, but allows all other traffic
The deny statement blocks port 23 (Telnet), and the permit any permits everything else.
B
Blocks SSH traffic (port 22) from entering the interface
Why wrong: SSH uses port 22, which is not blocked.
C
Blocks Telnet traffic leaving the interface, but allows all other traffic
Why wrong: The ACL is applied inbound (in), so it filters incoming traffic.
D
Blocks all TCP traffic except Telnet
Why wrong: It denies only Telnet (port 23) and permits all other TCP and IP traffic.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Blocks Telnet traffic entering the interface, but allows all other traffic
Option A is correct because the ACL denies TCP traffic to port 23 (Telnet) and permits all other IP traffic. Option B is wrong because SSH is port 22. Option C is wrong because the ACL is applied inbound on Gig0/1, so it filters incoming traffic only, not outgoing. Option D is wrong because it applies to all TCP traffic, not just Telnet.
Key principle: ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Blocks Telnet traffic entering the interface, but allows all other traffic
Why this is correct
The deny statement blocks port 23 (Telnet), and the permit any permits everything else.
Related concept
Standard ACLs match source addresses.
✗
Blocks SSH traffic (port 22) from entering the interface
Blocks Telnet traffic leaving the interface, but allows all other traffic
Why it's wrong here
The ACL is applied inbound (in), so it filters incoming traffic.
✗
Blocks all TCP traffic except Telnet
Why it's wrong here
It denies only Telnet (port 23) and permits all other TCP and IP traffic.
Common exam traps
Common exam trap: ACLs stop at the first match
ACLs are processed top to bottom. The first matching entry wins, and an implicit deny usually exists at the end.
Detailed technical explanation
How to think about this question
ACL questions test precision: source, destination, protocol, port and direction. A generally correct ACL can still fail if it is applied on the wrong interface or in the wrong direction.
KKey Concepts to Remember
Standard ACLs match source addresses.
Extended ACLs can match source, destination, protocol and ports.
The first matching ACL entry is used.
There is usually an implicit deny at the end.
TExam Day Tips
→Check inbound versus outbound direction.
→Read the ACL from top to bottom.
→Look for a broader permit or deny above the intended line.
Key takeaway
ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement.
Real-world example
How this comes up in practice
A security analyst at a medium-sized enterprise encounters this scenario during an investigation or architecture review. The correct answer reflects best practice for the specific threat or control described. ACLs process entries top to bottom and stop at the first match. Entry order and interface direction matter as much as the permit or deny statement. Security exam questions test whether you can match controls to threats in context — not just recall definitions.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this CC question in full detail.
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CC ACL questions on filtering logic and placement.
Security Principles — This question tests Security Principles — Standard ACLs match source addresses..
What is the correct answer to this question?
The correct answer is: Blocks Telnet traffic entering the interface, but allows all other traffic — Option A is correct because the ACL denies TCP traffic to port 23 (Telnet) and permits all other IP traffic. Option B is wrong because SSH is port 22. Option C is wrong because the ACL is applied inbound on Gig0/1, so it filters incoming traffic only, not outgoing. Option D is wrong because it applies to all TCP traffic, not just Telnet.
What should I do if I get this CC question wrong?
Review ACL processing order, placement rules (standard near destination, extended near source), and inbound vs outbound direction. Study wildcard masks and implicit deny. Then practise related CC ACL questions on filtering logic and placement.
What is the key concept behind this question?
Standard ACLs match source addresses.
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