hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures NAT overload (PAT) on a…
An engineer configures NAT overload (PAT) on a router to translate internal addresses to a single public IP. Users can browse the web, but some applications that use non-standard ports fail. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that PAT works for all IP traffic, when in fact it only supports TCP, UDP, and ICMP (with limitations) because it requires port numbers for multiplexing.
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 application uses protocols that do not have port numbers, such as GRE, and PAT cannot handle them.
PAT (Port Address Translation) relies on TCP/UDP port numbers to multiplex multiple internal addresses to a single public IP. Protocols like GRE (Generic Routing Encapsulation) and IPsec ESP/AH do not use port numbers, so PAT cannot differentiate between multiple sessions using these protocols. This causes the translation to fail for such applications, even though standard web traffic (HTTP/HTTPS) works fine.
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 application uses protocols that do not have port numbers, such as GRE, and PAT cannot handle them.
Why this is correct
Correct. PAT requires port numbers; non-TCP/UDP protocols fail.
- ✗
The NAT pool is exhausted.
Why it's wrong here
Exhaustion would affect all traffic, not just non-standard ports.
- ✗
The inside interface is not configured correctly.
Why it's wrong here
Interface misconfiguration would affect all traffic.
- ✗
The outside interface has a different MTU.
Why it's wrong here
MTU mismatch would cause fragmentation issues, not port-specific failure.
Visual reference
Quick reference
VPN Protocol Comparison
| Protocol | Port | Encryption | Authentication | Use Case |
|---|---|---|---|---|
| IKEv2 / IPsec | UDP 500 / 4500 | AES-256 | Certificates / PSK | Site-to-site & remote access |
| SSL / TLS VPN | TCP 443 | TLS 1.3 | Certificates / MFA | Clientless remote access |
| L2TP / IPsec | UDP 1701 | AES (IPsec) | PSK / Certificates | Legacy remote access |
| WireGuard | UDP 51820 | ChaCha20 | Public keys | Modern high-performance VPN |
| PPTP | TCP 1723 | MPPE (weak) | MS-CHAPv2 | Legacy — avoid in production |
PPTP is considered insecure. IKEv2/IPsec and SSL VPN are the current recommended options.
Go deeper
Related to this question
About these practice questions
Courseiva writes every 300-410 question from scratch — 1,966 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
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.