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.”
300-410 NAT and PAT Practice Question
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.
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.
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.
PAT uses port numbers to multiplex many inside hosts to a single outside IP. Each TCP/UDP session consumes a port; when the port range is exhausted, new translations cannot be created.
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 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
Static NAT maps one inside address to one outside address.
✗
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: 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 300-410 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 300-410 NAT questions on configuration and troubleshooting.
NAT and PAT — This question tests NAT and PAT — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The router has run out of available port numbers for PAT. — PAT uses port numbers to multiplex many inside hosts to a single outside IP. Each TCP/UDP session consumes a port; when the port range is exhausted, new translations cannot be created.
What should I do if I get this 300-410 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 300-410 NAT questions on configuration and troubleshooting.
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.
About these practice questions
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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 pool has only 14 addresses, and all are allocated (100%). With 1024 active translations, this indicates PAT is using all addresses and may be running out of ports.
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Question Discussion
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