This 200-301 practice question tests your understanding of ip routing. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. A key principle to apply: a router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.. 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 statement best explains why a router uses longest-prefix match before other route-comparison steps between matching routes?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Because the router first prefers the route that most precisely matches the destination.
Longest-prefix match comes first because route specificity is the most direct way to identify the best matching destination range. In plain language, if several routes could all technically match a packet, the router first asks which one describes the destination most precisely. Only after that does it compare other properties among routes of equal prefix length.
This is fundamental to routing-table logic. If the router ignored prefix specificity, detailed route entries would lose much of their value. The correct answer is the one that explains route choice in terms of the most specific valid match.
Key principle: A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
Because the router first prefers the route that most precisely matches the destination.
Why this is correct
This is correct because longest-prefix match is based on specificity.
Clue confirmation
The clue word "best" in the question point toward this answer.
Related concept
A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
✗
Because administrative distance is meaningful only for MAC addresses.
Why it's wrong here
This is wrong because administrative distance is a route-source trust concept, not a MAC-address concept.
When this WOULD be correct
In a question specifically about routing protocols and their administrative distances, such as 'Which statement about administrative distance is true?', option B could be correct if it stated that administrative distance is relevant for determining the preferred route among different protocols, but it would need to clarify that it applies to IP routing, not MAC addresses.
✗
Because default routes are considered more specific than host routes.
Why it's wrong here
This is wrong because default routes are the least specific, not the most specific.
When this WOULD be correct
In a different question that asks about route specificity in a context where default routes are misdefined or misunderstood, option C could be correct if the question inaccurately states that default routes are treated as more specific than host routes due to a specific routing protocol behavior.
✗
Because OSPF requires it only on Ethernet links.
Why it's wrong here
This is wrong because longest-prefix match is general routing logic, not an Ethernet-only OSPF rule.
When this WOULD be correct
In a different question that asks about OSPF behavior specifically on Ethernet links, and if it were framed to imply that OSPF has unique requirements for longest-prefix matching on Ethernet, this option could be correct. For instance, if the question stated that OSPF has special handling rules for Ethernet interfaces, then option D could be seen as valid.
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.
✓Because the router first prefers the route that most precisely matches the destination.Correct answer▾
Why this is correct
This is correct because longest-prefix match is based on specificity.
✗Because administrative distance is meaningful only for MAC addresses.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because administrative distance applies to routing protocols, not MAC addresses. Routers use administrative distance to determine which routing protocol to prefer when multiple routes exist, but this concept does not pertain to MAC addresses.
★ When this WOULD be the correct answer
In a question specifically about routing protocols and their administrative distances, such as 'Which statement about administrative distance is true?', option B could be correct if it stated that administrative distance is relevant for determining the preferred route among different protocols, but it would need to clarify that it applies to IP routing, not MAC addresses.
Why candidates choose this
Candidates may be tempted by this option due to a misunderstanding of administrative distance, thinking it relates to all types of addresses rather than just IP addresses in the context of routing protocols.
✗Because default routes are considered more specific than host routes.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because default routes are not more specific than host routes; in fact, host routes are the most specific type of route. The longest-prefix match prioritizes the most specific route, which would be a host route over a default route.
★ When this WOULD be the correct answer
In a different question that asks about route specificity in a context where default routes are misdefined or misunderstood, option C could be correct if the question inaccurately states that default routes are treated as more specific than host routes due to a specific routing protocol behavior.
Why candidates choose this
Candidates may choose this option due to a misunderstanding of route specificity, confusing the hierarchy of route types and mistakenly believing that default routes can be more specific than host routes in certain scenarios.
✗Because OSPF requires it only on Ethernet links.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because OSPF (Open Shortest Path First) does not impose any requirement for longest-prefix matching specifically on Ethernet links; it applies universally across all interfaces. Longest-prefix matching is a fundamental routing principle that applies to all routes, regardless of the underlying link type.
★ When this WOULD be the correct answer
In a different question that asks about OSPF behavior specifically on Ethernet links, and if it were framed to imply that OSPF has unique requirements for longest-prefix matching on Ethernet, this option could be correct. For instance, if the question stated that OSPF has special handling rules for Ethernet interfaces, then option D could be seen as valid.
Why candidates choose this
Candidates may find this option tempting because it references OSPF, a common routing protocol, and they might mistakenly associate specific behaviors with certain link types, leading to confusion about routing principles.
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: answer the scenario, not the keyword
A frequent exam trap is confusing longest-prefix match with administrative distance or metric as the first step in route selection. Candidates may incorrectly believe that administrative distance, which ranks route trustworthiness, is applied before prefix length. Another common mistake is thinking default routes are more specific than host routes, when in fact default routes have the shortest prefix and are the least specific. Misunderstanding these concepts can lead to incorrect answers about routing behavior. Remember, longest-prefix match always precedes other route-comparison steps to ensure the router selects the most precise route before considering other factors.
Detailed technical explanation
How to think about this question
Longest-prefix match is a fundamental routing principle where a router selects the route with the most specific subnet mask that matches the destination IP address. This means the route with the longest subnet prefix length (most bits set to 1) is preferred because it narrows down the destination network more precisely than broader routes. This specificity ensures that packets are forwarded along the most accurate path possible, reducing ambiguity in routing decisions.
When multiple routes to the same destination exist, the router first applies the longest-prefix match rule to filter out less specific routes. Only after identifying routes with the same prefix length does the router compare other attributes such as administrative distance and metric to determine the best path. This hierarchical decision process guarantees that route specificity takes precedence, which is critical in complex networks where overlapping routes may exist.
A common exam trap is misunderstanding the role of longest-prefix match versus administrative distance or metric. Some candidates incorrectly assume that administrative distance is the first criterion or that default routes are more specific than host routes. In practice, default routes have the shortest prefix (0.0.0.0/0) and are the least specific. Cisco routers always apply longest-prefix match first to ensure the most precise route is chosen before considering trustworthiness or cost metrics. This behavior is consistent across routing protocols and is essential for predictable routing outcomes.
KKey Concepts to Remember
A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
Longest-prefix match ensures packets are forwarded along the most precise path by preferring routes with longer subnet prefixes.
When multiple routes match a destination, the router first filters by prefix length before comparing administrative distance or metrics.
Default routes have the shortest prefix length and are considered the least specific routes in routing decisions.
Administrative distance is used only after longest-prefix match to choose between routes with equal prefix lengths.
Routing protocols like OSPF and EIGRP rely on longest-prefix match as a fundamental routing table lookup rule.
Ignoring longest-prefix match would cause routers to select less specific routes, reducing routing accuracy and efficiency.
Longest-prefix match is a universal routing principle applied regardless of the underlying routing protocol or interface type.
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
A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 200-301 question in full detail.
Review a router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
IP Routing — This question tests IP Routing — A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address..
What is the correct answer to this question?
The correct answer is: Because the router first prefers the route that most precisely matches the destination. — Longest-prefix match comes first because route specificity is the most direct way to identify the best matching destination range. In plain language, if several routes could all technically match a packet, the router first asks which one describes the destination most precisely. Only after that does it compare other properties among routes of equal prefix length.
This is fundamental to routing-table logic. If the router ignored prefix specificity, detailed route entries would lose much of their value. The correct answer is the one that explains route choice in terms of the most specific valid match.
What should I do if I get this 200-301 question wrong?
Review a router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address., then practise related 200-301 questions on the same topic to reinforce the concept.
Are there clue words in this question I should notice?
Yes — watch for: "best". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
A router uses longest-prefix match to select the route with the most specific subnet mask that matches the destination IP address.
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