mediumMultiple ChoiceObjective-mapped
PAT Port Exhaustion
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?
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.”
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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.
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Same concept, more angles
1 more way 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.