The correct answer is to configure the ACL with the sequence: permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25, then deny tcp any any eq 25, and finally permit ip any any. This order is critical because extended ACLs are processed top-down; the specific permit for the allowed SMTP traffic must come first so it is matched before the general deny for all other SMTP, and the final permit ip any any ensures all non-SMTP traffic is allowed. On the CCNA 200-301 v2 exam, this tests your understanding of extended ACL sequence logic and the implicit deny any rule—a common trap is placing a broad permit or deny statement too early, which either bypasses the restriction or blocks everything. For SMTP configuration order, remember the mnemonic "Specific, Deny, Allow All" to avoid the mistake of letting a permit ip any any appear before your deny statement, which would render the deny unreachable.
CCNA Network Services and Security Practice Question
This 200-301 practice question tests your understanding of network services and security. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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
R1# show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip access-group BLOCK_SMTP in
duplex auto
speed auto
!
R1# show access-lists BLOCK_SMTP
Extended IP access list BLOCK_SMTP
10 deny tcp any any eq 25
20 permit ip any any
You are connected to R1 via the console. An extended ACL named BLOCK_SMTP has been applied inbound on interface GigabitEthernet0/1, but users on the 192.168.10.0/24 network cannot send email to the SMTP server at 203.0.113.10. Additionally, the ACL is blocking all other traffic that should be permitted. Examine the running configuration and fix the ACL so that SMTP traffic (TCP port 25) from the 192.168.10.0/24 network to the SMTP server is permitted, and all other IP traffic is allowed.
R1# show running-config | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
ip address 192.168.10.1 255.255.255.0
ip access-group BLOCK_SMTP in
duplex auto
speed auto
!
R1# show access-lists BLOCK_SMTP
Extended IP access list BLOCK_SMTP
10 deny tcp any any eq 25
20 permit ip any any
A
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
deny tcp any any eq 25
permit ip any any
This sequence correctly permits SMTP from the specific source to the specific destination, then denies all other SMTP, and finally permits all other IP traffic. The order ensures that the desired SMTP traffic is allowed before the general deny, and the permit ip any any at the end allows all non-SMTP traffic.
B
permit tcp any any eq 25
deny tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
Why wrong: This is incorrect because it permits all SMTP traffic first, then denies the specific SMTP traffic, which has no effect since the permit any any already allows it. The deny line is never reached for that traffic.
C
deny tcp any any eq 25
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
Why wrong: This is incorrect because the deny tcp any any eq 25 will block all SMTP traffic, including the desired traffic from 192.168.10.0/24 to 203.0.113.10, before the permit line is evaluated.
D
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
deny tcp any any eq 25
Why wrong: This is incorrect because the permit ip any any at the end allows all IP traffic, including SMTP from other sources, before the deny tcp any any eq 25 is reached. The deny line becomes useless.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
deny tcp any any eq 25
permit ip any any
The correct answer is A. It permits SMTP from 192.168.10.0/24 to the SMTP server, then denies all other SMTP traffic, and finally permits all other IP traffic. Option B is wrong because it places a general permit for any SMTP before the specific deny, meaning all SMTP is permitted regardless of source/destination. Option C is wrong because it denies all SMTP first, so the subsequent specific permit for the user network is never reached. Option D is wrong because the order permits the specific SMTP, then permits all IP traffic (including other SMTP), then denies SMTP—the permit ip any any before the deny makes the deny unreachable for all traffic, allowing all SMTP.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
deny tcp any any eq 25
permit ip any any
Why this is correct
This sequence correctly permits SMTP from the specific source to the specific destination, then denies all other SMTP, and finally permits all other IP traffic. The order ensures that the desired SMTP traffic is allowed before the general deny, and the permit ip any any at the end allows all non-SMTP traffic.
Related concept
Static NAT maps one inside address to one outside address.
✗
permit tcp any any eq 25
deny tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
Why it's wrong here
This is incorrect because it permits all SMTP traffic first, then denies the specific SMTP traffic, which has no effect since the permit any any already allows it. The deny line is never reached for that traffic.
✗
deny tcp any any eq 25
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
Why it's wrong here
This is incorrect because the deny tcp any any eq 25 will block all SMTP traffic, including the desired traffic from 192.168.10.0/24 to 203.0.113.10, before the permit line is evaluated.
✗
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
deny tcp any any eq 25
Why it's wrong here
This is incorrect because the permit ip any any at the end allows all IP traffic, including SMTP from other sources, before the deny tcp any any eq 25 is reached. The deny line becomes useless.
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.
✓permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
deny tcp any any eq 25
permit ip any anyCorrect answer▾
Why this is correct
This sequence correctly permits SMTP from the specific source to the specific destination, then denies all other SMTP, and finally permits all other IP traffic. The order ensures that the desired SMTP traffic is allowed before the general deny, and the permit ip any any at the end allows all non-SMTP traffic.
✗permit tcp any any eq 25
deny tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any anyWrong answer — click to see why▾
Why this is wrong here
The order of ACL entries is crucial; the first match is applied. Here, the permit any any matches all SMTP before the deny can block the specific traffic.
Why candidates choose this
Candidates may think that placing the deny after the permit will override it, but ACLs are processed top-down and the first match wins.
✗deny tcp any any eq 25
permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any anyWrong answer — click to see why▾
Why this is wrong here
The deny any any matches all SMTP, so the subsequent permit for the specific source/destination is never reached.
Why candidates choose this
Candidates might think that a more specific permit can override a general deny, but ACLs are processed sequentially; the first match (deny) blocks the traffic.
✗permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
permit ip any any
deny tcp any any eq 25Wrong answer — click to see why▾
Why this is wrong here
The permit ip any any matches all traffic, so the subsequent deny for SMTP is never applied.
Why candidates choose this
Candidates might think that placing the deny at the end will block SMTP, but the permit ip any any already permits everything, including SMTP.
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
Network Services and Security — This question tests Network Services and Security — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: permit tcp 192.168.10.0 0.0.0.255 host 203.0.113.10 eq 25
deny tcp any any eq 25
permit ip any any — The correct answer is A. It permits SMTP from 192.168.10.0/24 to the SMTP server, then denies all other SMTP traffic, and finally permits all other IP traffic. Option B is wrong because it places a general permit for any SMTP before the specific deny, meaning all SMTP is permitted regardless of source/destination. Option C is wrong because it denies all SMTP first, so the subsequent specific permit for the user network is never reached. Option D is wrong because the order permits the specific SMTP, then permits all IP traffic (including other SMTP), then denies SMTP—the permit ip any any before the deny makes the deny unreachable for all traffic, allowing all SMTP.
What should I do if I get this 200-301 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 200-301 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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