Question 1hardmultiple choice
Full question →hardmultiple choiceObjective-mapped
A host with address 172.16.5.10/23 wants to determine whether 172.16.6.20 is on the same local network. What is the correct conclusion?
Answer choices
Why each option matters
Good practice is not just finding the correct option. The wrong answers often show the exact trap the exam wants you to fall into.
A
Distractor review
It is on the same local network because both addresses begin with 172.16.
This is wrong because sharing the first two octets is not enough to determine the subnet relationship.
B
Distractor review
It is on the same local network because /23 covers all addresses in 172.16.5.x and 172.16.6.x
This is wrong because 172.16.5.10/23 belongs to the 172.16.4.0/23 block, not a block covering 5.x and 6.x.
C
Best answer
It is on a different network, so the host should use the default gateway
This is correct because 172.16.6.20 is not in the same /23 network as 172.16.5.10.
D
Distractor review
It is on a different network, so ARP will resolve it across the router automatically
This is wrong because ARP is a local-link mechanism and does not resolve remote hosts across routers.
Common exam trap
Common exam trap: answer the scenario, not the keyword
A frequent exam trap is assuming that two IP addresses are on the same local network simply because they share the first two octets or appear numerically close. Many candidates overlook how subnet masks extend or reduce the network range beyond default classful boundaries. In this question, mistaking 172.16.6.20 as local because it is near 172.16.5.10 ignores that the /23 mask groups addresses in blocks of two /24s starting at even numbers. This leads to the false conclusion that the destination is local, causing confusion about ARP and routing behavior.
Technical deep dive
How to think about this question
Subnetting is a fundamental concept in IP networking that divides a larger network into smaller, manageable segments called subnets. Each subnet is defined by a subnet mask or prefix length, which determines the network and host portions of an IP address. In this question, the /23 prefix means the first 23 bits of the IP address define the network portion, allowing the subnet to cover two consecutive /24 networks. For example, a /23 subnet starting at 172.16.4.0 includes all addresses from 172.16.4.0 to 172.16.5.255. To determine if two IP addresses are on the same local network, a host applies its subnet mask to both addresses and compares the resulting network addresses. If they match, the addresses are on the same subnet; if not, they are on different networks. Here, 172.16.5.10/23 belongs to the 172.16.4.0/23 subnet, while 172.16.6.20 falls outside this range, indicating it is on a different subnet. Consequently, the host must forward traffic destined for 172.16.6.20 to its default gateway (router) for proper routing. A common exam trap is assuming that IP addresses sharing the first two octets or even adjacent third octet values are always on the same subnet. This mistake ignores the subnet mask's role in defining network boundaries. In practical Cisco networking, understanding subnet boundaries is critical for correct routing and ARP behavior. ARP only resolves addresses on the local subnet; for remote subnets, the host relies on the default gateway to forward packets, emphasizing the importance of accurate subnet calculations.
KKey Concepts to Remember
- A subnet mask defines the network and host portions of an IP address, determining the local subnet boundaries for routing decisions.
- A /23 subnet mask covers two consecutive /24 networks, effectively doubling the size of the local subnet compared to a /24 mask.
- To verify if two IP addresses are on the same subnet, a host applies its subnet mask to both addresses and compares the resulting network addresses.
- If the network addresses differ after masking, the destination is off-subnet, requiring the host to forward packets to its default gateway router.
- ARP operates only within the local subnet and cannot resolve IP addresses that belong to different subnets across routers.
- Hosts must use the default gateway to communicate with devices outside their local subnet, ensuring proper routing of traffic.
- Assuming IP addresses are local based solely on matching initial octets without considering the subnet mask leads to incorrect routing behavior.
- Cisco networking exams test understanding of subnetting beyond default classful boundaries, emphasizing precise subnet mask application.
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.
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More questions from this exam
Keep practising from the same exam bank, or move into a focused topic page if this question exposed a weak area.
Question 1
A router learns the same prefix from both OSPF and EIGRP. Which route is installed by default?
Question 2
A router shows this output: R1#show ip ospf neighbor Neighbor ID Pri State Dead Time Address Interface 10.1.1.2 1 FULL/DR 00:00:34 192.168.12.2 GigabitEthernet0/0 10.1.1.3 1 2WAY/DROTHER 00:00:39 192.168.12.3 GigabitEthernet0/0 Which statement is correct?
Question 3
What is the OSPF metric called?
Question 4
A non-root switch has two uplinks toward the root bridge. One path has a lower total STP cost than the other. What role will the lower-cost uplink have?
Question 5
A router interface applies this ACL inbound: 10 deny tcp any any eq 80 20 permit ip any any A user reports that web browsing to a server by IP address fails, but ping works. Which statement best explains the behavior?
Question 6
A router learns route 198.51.100.0/24 from OSPF with AD 110 and also has a static route to the same prefix configured with AD 150. Which route is installed?
FAQ
Questions learners often ask
What does this 200-301 question test?
A subnet mask defines the network and host portions of an IP address, determining the local subnet boundaries for routing decisions.
What is the correct answer to this question?
The correct answer is: It is on a different network, so the host should use the default gateway — A /23 prefix covers two consecutive /24 ranges. In plain language, that means the network boundaries move in blocks of 2 in the third octet. The address 172.16.5.10/23 belongs to the 172.16.4.0/23 network, which covers 172.16.4.x and 172.16.5.x. The destination 172.16.6.20 belongs to a different /23 block, so it is not on the same local network. This question checks whether you can think beyond default /24 boundaries and understand how a broader prefix changes the local network range. The host would therefore need to use its default gateway to reach 172.16.6.20, because that destination is off-subnet relative to the /23 in use.
What should I do if I get this 200-301 question wrong?
Then try more questions from the same exam bank and focus on understanding why the wrong options are tempting.
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