CCNA Network Infrastructure and Connectivity Practice Question
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?
⚠ Common exam trap
Don't assume addresses with the same initial octets are in the same subnet; always calculate based on the subnet mask.
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
✓
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
It is on the same local network because both addresses begin with 172.16.
Why it's wrong here
Merely sharing the first two octets is insufficient to conclude that two hosts are on the same subnet. The network boundary is determined by the /23 prefix length, which includes 23 bits of the IP address. Performing a bitwise AND of 172.16.5.10 with 255.255.254.0 yields 172.16.4.0, whereas the same operation on 172.16.6.20 yields 172.16.6.0. Because these network addresses differ, the hosts are on different networks despite both starting with 172.16.
When this WOULD be correct
If the question stated that both addresses were part of a larger subnet, such as 172.16.0.0/16, then this option would be correct, as both addresses would indeed be on the same local network.
- ✗
It is on the same local network because /23 covers all addresses in 172.16.5.x and 172.16.6.x
Why it's wrong here
The /23 prefix does not encompass both 172.16.5.x and 172.16.6.x as a single network. A /23 subnet must align on an even boundary in the third octet; the block containing .5.x is 172.16.4.0/23 (hosts 172.16.4.1–172.16.5.254), while .6.x falls in the next /23 block, 172.16.6.0/23. Thus 172.16.5.10 and 172.16.6.20 are in different /23 subnets, so this option's claim is false.
When this WOULD be correct
If the question stated that the subnet mask was /22 instead of /23, then option B would be correct, as a /22 subnet would encompass both 172.16.5.x and 172.16.6.x addresses, allowing them to be on the same local network.
- ✓
It is on a different network, so the host should use the default gateway
Why this is correct
The host determines the network address by performing a bitwise AND operation between its IP address (172.16.5.10) and its /23 subnet mask (255.255.254.0), resulting in 172.16.4.0. It then applies the same subnet mask to the target IP (172.16.6.20), yielding 172.16.6.0. Since these calculated network addresses differ, the host concludes the target is on a different network. Consequently, it must forward traffic to its default gateway for routing.
- ✗
It is on a different network, so ARP will resolve it across the router automatically
Why it's wrong here
ARP (Address Resolution Protocol) operates only within the local broadcast domain and cannot resolve IP addresses across a router. When a host determines that a destination is on a different network, it does not send an ARP request for that remote address; instead, it forwards the packet to the configured default gateway (router). The router then uses its own ARP table or ARP requests to reach the next hop toward the final destination. Therefore, ARP will not 'resolve across the router' automatically.
When this WOULD be correct
If the question stated that both addresses were on the same local network but required ARP to resolve a device's MAC address for communication, then option D would be correct. For example, if both addresses were within the same subnet and the question asked about resolving a MAC address, D would apply.
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.
✓It is on a different network, so the host should use the default gatewayCorrect answer▾
Why this is correct
The host determines the network address by performing a bitwise AND operation between its IP address (172.16.5.10) and its /23 subnet mask (255.255.254.0), resulting in 172.16.4.0. It then applies the same subnet mask to the target IP (172.16.6.20), yielding 172.16.6.0. Since these calculated network addresses differ, the host concludes the target is on a different network. Consequently, it must forward traffic to its default gateway for routing.
✗It is on the same local network because both addresses begin with 172.16.Wrong answer — click to see why▾
Why this is wrong here
This option is incorrect because the addresses 172.16.5.10 and 172.16.6.20 belong to different subnets; 172.16.6.20 is in the 172.16.6.0/23 subnet, while 172.16.5.10 is in the 172.16.5.0/23 subnet.
★ When this WOULD be the correct answer
If the question stated that both addresses were part of a larger subnet, such as 172.16.0.0/16, then this option would be correct, as both addresses would indeed be on the same local network.
Why candidates choose this
Candidates may be drawn to this option because it highlights a common misconception that addresses starting with the same octets are always in the same network, ignoring the significance of the subnet mask.
✗It is on the same local network because /23 covers all addresses in 172.16.5.x and 172.16.6.xWrong answer — click to see why▾
Why this is wrong here
This option is incorrect because a /23 subnet mask allows for addresses in the range of 172.16.4.0 to 172.16.5.255, meaning 172.16.6.20 is outside this range and not on the same local network.
★ When this WOULD be the correct answer
If the question stated that the subnet mask was /22 instead of /23, then option B would be correct, as a /22 subnet would encompass both 172.16.5.x and 172.16.6.x addresses, allowing them to be on the same local network.
Why candidates choose this
Candidates may be tempted by this option because they recognize that both addresses share the same initial octets, leading them to mistakenly assume they are in the same subnet without considering the subnet mask.
✗It is on a different network, so ARP will resolve it across the router automaticallyWrong answer — click to see why▾
Why this is wrong here
Option D is incorrect because ARP (Address Resolution Protocol) operates within the same local network and cannot resolve addresses across routers. Since 172.16.6.20 is on a different network than 172.16.5.10, ARP cannot automatically resolve it.
★ When this WOULD be the correct answer
If the question stated that both addresses were on the same local network but required ARP to resolve a device's MAC address for communication, then option D would be correct. For example, if both addresses were within the same subnet and the question asked about resolving a MAC address, D would apply.
Why candidates choose this
Candidates might choose this option due to a misunderstanding of ARP's functionality, believing it can resolve addresses across different networks, which is a common misconception among those with limited networking knowledge.
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?”
Visual reference
Quick reference
IPv4 Address Class Summary
| Class | First Octet Range | Default Mask | Networks | Hosts per Network |
|---|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | 126 | 16,777,214 |
| B | 128–191 | /16 (255.255.0.0) | 16,384 | 65,534 |
| C | 192–223 | /24 (255.255.255.0) | 2,097,152 | 254 |
| D | 224–239 | N/A | Multicast groups | — |
| E | 240–255 | N/A | Reserved / experimental | — |
127.x.x.x is reserved for loopback. Modern networks use CIDR (classless) rather than classful addressing.
Go deeper
Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
Key term
Default gateway
A default gateway is a network device, typically a router, that acts as the exit point for traffic from a local network to other networks, including the internet.
About these practice questions
One of 1,389 original 200-301 practice questions on Courseiva, each with a full explanation and wrong-answer analysis — not exam dumps or protected exam content. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 200-301 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 200-301 exam.