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CCNA Practice Question: Which TWO statements correctly describe the…
This 200-301 practice question tests your understanding of 200-301 exam topics. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which TWO statements correctly describe the encapsulation and de-encapsulation process at the transport layer of the OSI model?
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
✓
During encapsulation, the transport layer adds a header containing source and destination port numbers.
In the OSI model, the transport layer (Layer 4) is responsible for end-to-end communication and uses segments (TCP) or datagrams (UDP) as its PDU. During encapsulation, the transport layer adds a header containing source and destination port numbers to the data received from the session layer. During de-encapsulation, the receiving device's transport layer removes this header to extract the payload for the session layer. Options B and D accurately describe these processes. Option A is incorrect because the PDU at the transport layer is a segment (or datagram), not a packet (which is Layer 3). Option C describes a Layer 3 (network layer) function, and Option E describes a Layer 2 (data link layer) function.
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.
- ✗
During encapsulation, the transport layer adds a header to form a packet.
- ✓
During encapsulation, the transport layer adds a header containing source and destination port numbers.
Why this is correct
The transport layer header includes port numbers to identify the application processes on the source and destination hosts. This is a key part of encapsulation.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
During de-encapsulation, the transport layer removes the IP header to extract the segment.
Why it's wrong here
Removing the IP header is a function of the network layer (Layer 3) during de-encapsulation. The transport layer processes the segment header, not the IP header.
- ✓
During de-encapsulation, the transport layer removes its header and passes the payload to the session layer.
- ✗
During encapsulation, the transport layer adds a trailer for error detection.
Why it's wrong here
Error detection trailers (like CRC) are typically added at the data link layer (Layer 2), not at the transport layer. The transport layer may include a checksum in the header, but not a trailer.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓During encapsulation, the transport layer adds a header containing source and destination port numbers.Correct answer▾
Why this is correct
The transport layer header includes port numbers to identify the application processes on the source and destination hosts. This is a key part of encapsulation.
✗During encapsulation, the transport layer adds a header to form a packet.Wrong answer — click to see why▾
Why this is wrong here
This statement confuses the PDU naming: packet belongs to Layer 3, not Layer 4.
✗During de-encapsulation, the transport layer removes the IP header to extract the segment.Wrong answer — click to see why▾
Why this is wrong here
This describes a Layer 3 de-encapsulation step, not a Layer 4 step.
✗During encapsulation, the transport layer adds a trailer for error detection.Wrong answer — click to see why▾
Why this is wrong here
Trailers are a Layer 2 feature; the transport layer uses header-based checksums for integrity.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
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 200-301 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 200-301 question test?
Static NAT maps one inside address to one outside address.
What is the correct answer to this question?
The correct answer is: During encapsulation, the transport layer adds a header containing source and destination port numbers. — In the OSI model, the transport layer (Layer 4) is responsible for end-to-end communication and uses segments (TCP) or datagrams (UDP) as its PDU. During encapsulation, the transport layer adds a header containing source and destination port numbers to the data received from the session layer. During de-encapsulation, the receiving device's transport layer removes this header to extract the payload for the session layer. Options B and D accurately describe these processes. Option A is incorrect because the PDU at the transport layer is a segment (or datagram), not a packet (which is Layer 3). Option C describes a Layer 3 (network layer) function, and Option E describes a Layer 2 (data link layer) function.
What should I do if I get this 200-301 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 200-301 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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