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300-410 Practice Question: In Cisco IOS, what is the default behavior of the…
In Cisco IOS, what is the default behavior of the ip nat service command?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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