Question 1,395 of 2,152

NAT and PAT →mediumMultiple ChoiceObjective-mapped

PAT Port Exhaustion

This 300-410 practice question tests your understanding of nat and pat. 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 troubleshooting PAT (overload) on a Cisco router. The inside network uses 192.168.1.0/24, and the outside interface has IP 198.51.100.1. The engineer configured 'ip nat inside source list 1 interface GigabitEthernet0/0 overload'. Traffic from inside hosts works initially, but after a few minutes, new connections fail. 'Show ip nat translations' shows many entries with the same outside global IP but different ports. 'Show ip nat statistics' indicates that the number of translations is near 500. 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.

A
The NAT pool is not configured with overload.
Why wrong: Incorrect because overload is configured, as seen by the many translations sharing the same IP.
B
The outside interface is flapping, causing translations to be cleared.
Why wrong: Incorrect because if the interface flapped, translations would be cleared, not accumulated.
C
The router has run out of available port numbers for PAT.
Correct because PAT uses a limited port range (usually 1024-65535), and with many sessions, ports can be exhausted, preventing new translations.
D
The access list is denying some inside hosts.
Why wrong: Incorrect because if the ACL were denying hosts, those hosts would never get translations; the issue is that translations exist but new ones fail.

Full breakdown with real-world context →

Quick Answer

The answer is that the router has exhausted the available port numbers for PAT, preventing new translations from being created. PAT, or Port Address Translation, multiplexes multiple inside hosts to a single outside IP by assigning a unique source port to each session; with only 65,536 ports available per IP (minus reserved ranges), and each TCP or UDP flow consuming one, hitting the limit—often near 500 in Cisco’s default configuration—blocks new connections. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of NAT overload behavior and the finite port resource, a common trap where engineers overlook that PAT exhaustion, not ACL misconfiguration or routing issues, causes intermittent failures after initial success. Remember the memory tip: “PAT ports are like parking spots—once full, no new cars can park.”

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 router has run out of available port numbers for PAT.

The router is using PAT (Port Address Translation) with overload, which maps multiple inside local IP addresses to a single outside global IP (198.51.100.1) by using unique source port numbers. With approximately 500 active translations and the router nearing the default limit of around 500 PAT entries (or the available port range of 1024–65535 being exhausted), new connections fail because no unique port numbers are available to assign. This is the classic symptom of PAT port exhaustion.

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.

✗
The NAT pool is not configured with overload.
Why it's wrong here
Incorrect because overload is configured, as seen by the many translations sharing the same IP.
✗
The outside interface is flapping, causing translations to be cleared.
Why it's wrong here
Incorrect because if the interface flapped, translations would be cleared, not accumulated.
✓
The router has run out of available port numbers for PAT.
Why this is correct
Correct because PAT uses a limited port range (usually 1024-65535), and with many sessions, ports can be exhausted, preventing new translations.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The access list is denying some inside hosts.
Why it's wrong here
Incorrect because if the ACL were denying hosts, those hosts would never get translations; the issue is that translations exist but new ones fail.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between NAT pool exhaustion (running out of IP addresses) and PAT port exhaustion (running out of port numbers), and candidates may mistakenly think the problem is with the access list or interface stability when the real issue is the finite number of available PAT ports.

Detailed technical explanation

How to think about this question

PAT uses a single public IP address and multiplexes sessions by assigning unique Layer 4 port numbers (typically from 1024 to 65535) for each translation. The router maintains a NAT translation table with a maximum size (often 500 by default on many Cisco IOS platforms), and once all available port numbers are consumed or the table is full, the router cannot create new translations, causing new connections to fail. This is a common issue in environments with many concurrent sessions, such as large enterprise networks or during DDoS attacks, and can be mitigated by increasing the NAT translation timeout or using multiple public IPs.

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

Quick reference

IPv4 Address Class Summary

Class	First Octet Range	Default Mask	Networks	Hosts per Network
A	1–126	/8 (255.0.0.0)	126	16,777,214
B	128–191	/16 (255.255.0.0)	16,384	65,534
C	192–223	/24 (255.255.255.0)	2,097,152	254
D	224–239	N/A	Multicast groups	—
E	240–255	N/A	Reserved / experimental	—

127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

Related 300-410 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Layer 3 Technologies practice questions

Practise 300-410 questions linked to Layer 3 Technologies.

EIGRP Troubleshooting practice questions

Practise 300-410 questions linked to EIGRP Troubleshooting.

OSPF Troubleshooting (v2/v3) practice questions

Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).

BGP Troubleshooting practice questions

Practise 300-410 questions linked to BGP Troubleshooting.

Route Redistribution practice questions

Practise 300-410 questions linked to Route Redistribution.

Policy-Based Routing (PBR) practice questions

Practise 300-410 questions linked to Policy-Based Routing (PBR).

VRF-Lite practice questions

Practise 300-410 questions linked to VRF-Lite.

Route Maps and Route Filtering practice questions

Practise 300-410 questions linked to Route Maps and Route Filtering.

Administrative Distance practice questions

Practise 300-410 questions linked to Administrative Distance.

Route Summarization practice questions

Practise 300-410 questions linked to Route Summarization.

Bidirectional Forwarding Detection (BFD) practice questions

Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).

VPN Technologies practice questions

Practise 300-410 questions linked to VPN Technologies.

Practice this exam

Start a free 300-410 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

300-410 practice-test guide →Study guide →Browse all practice tests

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: The router has run out of available port numbers for PAT. — The router is using PAT (Port Address Translation) with overload, which maps multiple inside local IP addresses to a single outside global IP (198.51.100.1) by using unique source port numbers. With approximately 500 active translations and the router nearing the default limit of around 500 PAT entries (or the available port range of 1024–65535 being exhausted), new connections fail because no unique port numbers are available to assign. This is the classic symptom of PAT port exhaustion.

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.

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?

Read the scenario before looking for a memorised answer.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer runs the following command to troubleshoot PAT exhaustion: R1# show ip nat statistics Total active translations: 1024 (0 static, 1024 dynamic; 1024 extended) Outside interfaces: GigabitEthernet0/1 Inside interfaces: GigabitEthernet0/0 Hits: 50000 Misses: 10 CEF Translated packets: 45000, CEF Punted packets: 5000 Expired translations: 2000 Dynamic mappings: -- Inside Source [Id: 1] access-list NAT permit ip 10.0.0.0 0.255.255.255 any refcount 1024, pool MyPool pool MyPool: netmask 255.255.255.240 start 203.0.113.1 end 203.0.113.14 type generic, total addresses 14, allocated 14 (100%), misses 0 What is the most likely issue?

hard

✓ A.The pool is exhausted; PAT is using all addresses, but port exhaustion may occur.
B.The access list is misconfigured, blocking traffic.
C.The outside interface is down.
D.Static translations are missing.

Why A: The output shows that the NAT pool 'MyPool' has 14 addresses, all of which are allocated (100% usage), and there are 1024 active translations. With only 14 public IPs, PAT can theoretically support up to 14 * 65535 = 917,490 ports, but the pool exhaustion indicates that all addresses are in use, and the high number of translations suggests that port exhaustion is imminent or occurring, as each address can only handle a finite number of simultaneous sessions before ports are exhausted.

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.