- A
It disables NAT for all protocols by default.
Why wrong: NAT is enabled for all protocols by default.
- B
It enables NAT only for TCP and UDP by default.
Why wrong: NAT supports all IP protocols by default.
- C
It is used to configure NAT for specific application-layer protocols and is not enabled by default.
Correct. The ip nat service command is used to fine-tune NAT for specific protocols and is not enabled by default; it is applied only when configured.
- D
It configures the NAT translation timeout for all services.
Why wrong: The ip nat translation timeout command is used for that purpose.
Question 477 of 2,152
Default Behavior of ip nat service Command
This 300-410 practice question tests your understanding of nat and pat. 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.
In Cisco IOS, what is the default behavior of the ip nat service command?
Quick Answer
The correct answer is that the ip nat service command is not enabled by default, meaning NAT is automatically active for all application-layer protocols without any additional configuration. This is because Cisco IOS NAT operates in a transparent, protocol-agnostic manner by default, handling IP address translation for any traffic that matches an access list or route map. The ip nat service command is specifically used to override this default behavior for certain protocols—such as SIP or H.323—by either disabling NAT for them or altering the port numbers they use, which is necessary when application-layer gateways or inspection engines interfere with translation. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of NAT’s default state versus its fine-tuning for voice or video protocols, and a common trap is assuming the command must be applied to enable NAT at all. Remember the memory tip: “Default is all; service is specific”—NAT works on everything out of the box, and ip nat service only steps in when you need to tweak a particular protocol.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
It is used to configure NAT for specific application-layer protocols and is not enabled by default.
The `ip nat service` command in Cisco IOS is used to enable NAT for specific application-layer protocols (e.g., SIP, H.323, DNS) that embed IP addresses in their payloads, requiring the NAT router to perform deep packet inspection. By default, this command is not enabled; NAT only translates the IP header for TCP and UDP traffic without application-layer awareness. Option C correctly identifies that the command is not enabled by default and is used for specific protocol configuration.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 disables NAT for all protocols by default.
Why it's wrong here
NAT is enabled for all protocols by default.
- ✗
It enables NAT only for TCP and UDP by default.
Why it's wrong here
NAT supports all IP protocols by default.
- ✓
It is used to configure NAT for specific application-layer protocols and is not enabled by default.
Why this is correct
Correct. The ip nat service command is used to fine-tune NAT for specific protocols and is not enabled by default; it is applied only when configured.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
It configures the NAT translation timeout for all services.
Why it's wrong here
The ip nat translation timeout command is used for that purpose.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume NAT automatically supports all protocols, but Cisco tests the distinction that only TCP and UDP are translated by default, while application-layer protocols require explicit `ip nat service` configuration.
Trap categories for this question
Command / output trap
The ip nat translation timeout command is used for that purpose.
Detailed technical explanation
How to think about this question
The `ip nat service` command leverages Cisco's NAT Application Layer Gateway (ALG) to rewrite IP addresses and ports within application payloads, such as modifying SIP Via headers or H.323 call signaling. Without this command, NAT will fail for protocols that carry IP addresses in the data portion, leading to one-way audio in VoIP or broken multimedia sessions. In real-world scenarios, enabling `ip nat service sip` is critical for SIP trunking across NAT boundaries.
KKey Concepts to Remember
- Read the scenario before looking for a memorised answer.
- Find the constraint that changes the correct option.
- Eliminate answers that are true in general but not in this case.
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
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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.
Visual reference
What to study next
Got this wrong? Here's your next step.
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NAT and PAT — study guide chapter
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FAQ
Questions learners often ask
What does this 300-410 question test?
NAT and PAT — This question tests NAT and PAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: It is used to configure NAT for specific application-layer protocols and is not enabled by default. — The `ip nat service` command in Cisco IOS is used to enable NAT for specific application-layer protocols (e.g., SIP, H.323, DNS) that embed IP addresses in their payloads, requiring the NAT router to perform deep packet inspection. By default, this command is not enabled; NAT only translates the IP header for TCP and UDP traffic without application-layer awareness. Option C correctly identifies that the command is not enabled by default and is used for specific protocol configuration.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Last reviewed: Jul 4, 2026
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