The answer is DSCP, the Differentiated Services Code Point field in the IP header. This 6-bit value is the correct choice for marking voice traffic because it allows routers to classify packets into distinct service classes, with voice traffic typically assigned DSCP 46 (Expedited Forwarding, or EF) to guarantee low latency and jitter. On the CompTIA Network+ N10-009 exam, this question tests your understanding of QoS marking fields, often contrasting DSCP with the older IP Precedence or the Layer 2 CoS bits; a common trap is confusing DSCP with the entire ToS byte, but remember DSCP uses only six of those eight bits. For the exam, a solid memory tip is that “DSCP EF 46” is the gold standard for voice—think “EF” for “Expedite First” to prioritize call quality over data.
N10-009 Network Implementation Practice Question
This N10-009 practice question tests your understanding of network implementation. 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.
A network administrator is configuring Quality of Service (QoS) on a router to prioritize voice traffic. Which of the following fields should be used to mark packets for classification and prioritization?
Why wrong: Source IP address can be used to classify traffic but does not mark the packet; marking is done by setting a specific field so downstream devices can prioritize based on the mark, not the original source.
B
DSCP
DSCP is a 6-bit field in the IP header used for packet classification and prioritization in QoS. It allows consistent priority handling across the network.
C
Source port number
Why wrong: Port numbers are transport layer identifiers; they can be used for classification but not for marking per se; DSCP is the standard marking method.
D
MAC address
Why wrong: MAC addresses are used at Layer 2 for local delivery and are not suitable for marking packets for end-to-end QoS.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
DSCP
DSCP (Differentiated Services Code Point) is the correct field because it is a 6-bit value in the IP header used to mark packets for QoS classification and prioritization, as defined in RFC 2474. Voice traffic typically uses DSCP EF (Expedited Forwarding, value 46) to ensure low latency and jitter, making it the standard choice for QoS marking on routers.
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.
✗
Source IP address
Why it's wrong here
Source IP address can be used to classify traffic but does not mark the packet; marking is done by setting a specific field so downstream devices can prioritize based on the mark, not the original source.
✓
DSCP
Why this is correct
DSCP is a 6-bit field in the IP header used for packet classification and prioritization in QoS. It allows consistent priority handling across the network.
Related concept
Read the scenario before looking for a memorised answer.
✗
Source port number
Why it's wrong here
Port numbers are transport layer identifiers; they can be used for classification but not for marking per se; DSCP is the standard marking method.
✗
MAC address
Why it's wrong here
MAC addresses are used at Layer 2 for local delivery and are not suitable for marking packets for end-to-end QoS.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse classification (using source IP, port, or MAC to identify traffic) with marking (setting a QoS field like DSCP or CoS), leading them to choose a valid classification method instead of the actual marking field required by the question.
Detailed technical explanation
How to think about this question
DSCP is placed in the ToS (Type of Service) byte of the IPv4 header, with the first 6 bits defining the per-hop behavior (PHB) and the remaining 2 bits for ECN (Explicit Congestion Notification). In real-world deployments, voice traffic is marked with DSCP EF (46) at the trust boundary (e.g., IP phone or switch), and the router uses this marking to apply strict priority queuing, ensuring voice packets are serviced before data packets even during congestion.
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 practitioner preparing for the N10-009 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this N10-009 question in full detail.
Network Implementation — This question tests Network Implementation — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: DSCP — DSCP (Differentiated Services Code Point) is the correct field because it is a 6-bit value in the IP header used to mark packets for QoS classification and prioritization, as defined in RFC 2474. Voice traffic typically uses DSCP EF (Expedited Forwarding, value 46) to ensure low latency and jitter, making it the standard choice for QoS marking on routers.
What should I do if I get this N10-009 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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