Question 526 of 2,015

NAT and DHCP →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the NAT translation table is filling up due to a large number of concurrent sessions, causing new translations to be denied. This is the classic symptom of PAT port exhaustion, where the router’s overload mechanism runs out of unique port numbers to map internal sessions to the single public IP address. On the ENCOR 350-401 exam, this scenario tests your understanding of how NAT overload (PAT) multiplexes traffic using source ports, and the common trap is confusing a full translation table with a routing or ACL issue—remember, PAT fails silently when the port pool is depleted, not when the public IP is “used up.” A quick memory tip: think of PAT like a hotel with one phone line—once every extension (port) is in use, new callers get a busy signal.

CCNP NAT and DHCP Practice Question

This 350-401 practice question tests your understanding of nat and dhcp. 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.

A network engineer is configuring NAT overload (PAT) on a Cisco router to allow multiple internal hosts to share a single public IP address. The engineer uses the command ip nat inside source list 1 interface GigabitEthernet0/0 overload. After testing, internal hosts can access the internet, but some applications fail intermittently. The engineer suspects a NAT issue. What is the most likely cause?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice

Read the full NAT/PAT explanation →

A
The access list 1 is too permissive and includes the public IP address of the router.
Why wrong: Incorrect because including the public IP in the access list would not cause intermittent failures; it might cause unnecessary translations but not port exhaustion.
B
The NAT translation table is filling up due to a large number of concurrent sessions, causing new translations to be denied.
Correct because PAT has a limited number of available port numbers (approximately 65,000 per public IP), and if many sessions are active, the table can become full, dropping new connections.
C
The router is not configured with ip nat inside on the internal interface.
Why wrong: Incorrect because if the inside interface were missing the command, no translations would occur at all, not just intermittent failures.
D
The overload keyword is misspelled or not supported on this IOS version.
Why wrong: Incorrect because the overload keyword is standard for PAT; a misspelling would cause a syntax error, not intermittent failures.

Full breakdown with real-world context →

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

✓

The NAT translation table is filling up due to a large number of concurrent sessions, causing new translations to be denied.

PAT uses port numbers to multiplex multiple sessions over a single public IP. If the port range is exhausted or if the NAT translation table is full, new sessions will fail.

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.

✗
The access list 1 is too permissive and includes the public IP address of the router.
Why it's wrong here
Incorrect because including the public IP in the access list would not cause intermittent failures; it might cause unnecessary translations but not port exhaustion.
✓
The NAT translation table is filling up due to a large number of concurrent sessions, causing new translations to be denied.
Why this is correct
Correct because PAT has a limited number of available port numbers (approximately 65,000 per public IP), and if many sessions are active, the table can become full, dropping new connections.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The router is not configured with ip nat inside on the internal interface.
Why it's wrong here
Incorrect because if the inside interface were missing the command, no translations would occur at all, not just intermittent failures.
✗
The overload keyword is misspelled or not supported on this IOS version.
Why it's wrong here
Incorrect because the overload keyword is standard for PAT; a misspelling would cause a syntax error, not intermittent failures.

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.

Trap categories for this question

Keyword trap
Incorrect because the overload keyword is standard for PAT; a misspelling would cause a syntax error, not intermittent failures.
Command / output trap
Incorrect because if the inside interface were missing the command, no translations would occur at all, not just intermittent failures.

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.

What to study next

Got this wrong? Here's your next step.

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 350-401 NAT questions on configuration and troubleshooting.

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FAQ

Questions learners often ask

What does this 350-401 question test?

NAT and DHCP — This question tests NAT and DHCP — Static NAT maps one inside address to one outside address..

What is the correct answer to this question?

The correct answer is: The NAT translation table is filling up due to a large number of concurrent sessions, causing new translations to be denied. — PAT uses port numbers to multiplex multiple sessions over a single public IP. If the port range is exhausted or if the NAT translation table is full, new sessions will fail.

What should I do if I get this 350-401 question wrong?

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Static NAT maps one inside address to one outside address.

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