The answer is to correct R1’s IPv4 subnet mask to 255.255.255.252, assign R2 the static IPv6 address 2001:db8:1::2/64, and configure a default route on R2 pointing to 192.0.2.1. This is correct because a point-to-point link between routers requires a /30 mask to avoid overlapping subnets—R1’s original /28 included both .1 and .14, breaking connectivity—while dual-stack IPv4/IPv6 addressing demands a static IPv6 address on R2 since R1’s EUI-64 address was valid but R2 had none. On the CCNA 200-301 v2 exam, this tests your ability to fix IPv4 subnet mask errors and configure IPv6 addressing in a dual-stack scenario, a common lab simulation trap where candidates forget that /30 is the standard for serial or Ethernet links between routers. A key memory tip: for point-to-point links, always think “/30 is the magic number”—it gives exactly two usable hosts and prevents subnet overlap.
CCNA Network Infrastructure and Connectivity Practice Question
This 200-301 practice question tests your understanding of network infrastructure and connectivity. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
Network Topology
You are connected to R1 via console. R1 and R2 are directly connected via GigabitEthernet0/0. Your task is to configure IPv4 and IPv6 addressing on both routers so that they can ping each other's IPv4 and IPv6 addresses. The current configuration has intentional faults: R1's IPv4 subnet mask is incorrect, R2 is missing its default gateway, and R1's IPv6 address uses EUI-64 but is not working due to a duplicate IP. Correct the IPv4 mask on R1, assign a static IPv6 address on R2, and ensure both routers can reach each other.
R1#show running-config | section interface GigabitEthernet0/0
interface GigabitEthernet0/0
ip address 192.0.2.1 255.255.255.240
ipv6 address 2001:db8:1::/64 eui-64
no shutdown
R2#show running-config | section interface GigabitEthernet0/0
interface GigabitEthernet0/0
ip address 192.0.2.14 255.255.255.240
no ipv6 address
no shutdown
R2#show ip route
Codes: L - local, C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2
ia - IS-IS inter area, * - candidate default, U - per-user static route
o - ODR, P - periodic downloaded static route, H - NHRP, l - LISP
a - application route
+ - replicated route, % - next hop override
Gateway of last resort is not set
C 192.0.2.0/28 is directly connected, GigabitEthernet0/0
R1#show ipv6 interface brief
GigabitEthernet0/0 [up/up]
FE80::/10
2001:DB8:1:0:20C:29FF:FE8C:1234
R2#ping 192.0.2.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.0.2.1, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R1#ping 192.0.2.14
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.0.2.14, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
R2#ping 2001:DB8:1:0:20C:29FF:FE8C:1234
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 2001:DB8:1:0:20C:29FF:FE8C:1234, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
A
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.
This is correct because the /30 mask (255.255.255.252) provides exactly two usable host addresses for a point-to-point link, avoiding overlap with other subnets. R2 needs a static IPv6 address in the same subnet as R1's EUI-64 address, and a default route to reach R1's IPv4 address.
B
On R1, change the IPv4 mask to 255.255.255.0; on R2, assign IPv4 address 192.0.2.2/24 and IPv6 address 2001:db8:1::2/64; no default route needed.
Why wrong: This is incorrect because a /24 mask would include many more addresses than needed, potentially overlapping with other subnets. Also, R2 needs a default route to reach R1's IPv4 address since they are on different subnets if the mask is /24? Actually, with /24 both are in the same subnet, but the mask is too large and not optimal for a point-to-point link. The question states R2 is missing its default gateway, so a default route is required.
C
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/28 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.
Why wrong: This is incorrect because R2's IPv4 mask should be /30 to match R1's corrected mask. Using /28 would put R2 in a different subnet (192.0.2.0/28 includes .1 and .14, but .2 is also in that subnet? Actually 192.0.2.0/28 includes .0 to .15, so .2 is in the same subnet as .1, but the mask mismatch would cause R2 to think the network is larger than it is, leading to potential routing issues. The correct mask is /30.
D
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::1/64; configure a default route on R2 pointing to 192.0.2.1.
Why wrong: This is incorrect because R2's IPv6 address should be different from R1's IPv6 address. R1 already has an EUI-64 address in the 2001:db8:1::/64 subnet, so assigning 2001:db8:1::1/64 to R2 would cause a duplicate IP address conflict.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.
R1's IPv4 mask was /28, but the correct mask for the link should be /30 to avoid overlapping subnets (192.0.2.0/28 includes both .1 and .14, but they are on the same link). R2 had no IPv6 address configured. Additionally, R1's EUI-64 address was valid but R2 needed a static IPv6 address. The solution: on R1, change the mask to 255.255.255.252; on R2, assign an IPv4 address with mask /30 and a static IPv6 address 2001:db8:1::2/64; also add a default route on R2 pointing to 192.0.2.1 for IPv4. After these changes, both routers can ping each other's IPv4 and IPv6 addresses.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.
Why this is correct
This is correct because the /30 mask (255.255.255.252) provides exactly two usable host addresses for a point-to-point link, avoiding overlap with other subnets. R2 needs a static IPv6 address in the same subnet as R1's EUI-64 address, and a default route to reach R1's IPv4 address.
On R1, change the IPv4 mask to 255.255.255.0; on R2, assign IPv4 address 192.0.2.2/24 and IPv6 address 2001:db8:1::2/64; no default route needed.
Why it's wrong here
This is incorrect because a /24 mask would include many more addresses than needed, potentially overlapping with other subnets. Also, R2 needs a default route to reach R1's IPv4 address since they are on different subnets if the mask is /24? Actually, with /24 both are in the same subnet, but the mask is too large and not optimal for a point-to-point link. The question states R2 is missing its default gateway, so a default route is required.
✗
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/28 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.
Why it's wrong here
This is incorrect because R2's IPv4 mask should be /30 to match R1's corrected mask. Using /28 would put R2 in a different subnet (192.0.2.0/28 includes .1 and .14, but .2 is also in that subnet? Actually 192.0.2.0/28 includes .0 to .15, so .2 is in the same subnet as .1, but the mask mismatch would cause R2 to think the network is larger than it is, leading to potential routing issues. The correct mask is /30.
✗
On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::1/64; configure a default route on R2 pointing to 192.0.2.1.
Why it's wrong here
This is incorrect because R2's IPv6 address should be different from R1's IPv6 address. R1 already has an EUI-64 address in the 2001:db8:1::/64 subnet, so assigning 2001:db8:1::1/64 to R2 would cause a duplicate IP address conflict.
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.
✓On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.Correct answer▾
Why this is correct
This is correct because the /30 mask (255.255.255.252) provides exactly two usable host addresses for a point-to-point link, avoiding overlap with other subnets. R2 needs a static IPv6 address in the same subnet as R1's EUI-64 address, and a default route to reach R1's IPv4 address.
✗On R1, change the IPv4 mask to 255.255.255.0; on R2, assign IPv4 address 192.0.2.2/24 and IPv6 address 2001:db8:1::2/64; no default route needed.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: Using a /24 mask on a point-to-point link wastes addresses and may cause subnet overlap; also, R2 needs a default route to reach R1's IPv4 address if the mask is /30, but with /24 they are in the same subnet so no default route is needed, but the mask is still wrong.
Why candidates choose this
Candidates might think a /24 mask is standard for Ethernet links, but for point-to-point links, a /30 is more appropriate.
✗On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/28 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: R2's IPv4 mask must match R1's mask to ensure both routers agree on the subnet boundary. Using /28 on R2 while R1 uses /30 creates a mismatch.
Why candidates choose this
Candidates might think R2 can use a different mask as long as the IP addresses are in the same range, but mismatched masks cause connectivity issues.
✗On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::1/64; configure a default route on R2 pointing to 192.0.2.1.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error: Assigning the same IPv6 address to both routers causes a duplicate address conflict, preventing communication.
Why candidates choose this
Candidates might mistakenly think that both routers can share the same IPv6 address if they are on different interfaces, but on the same link, addresses must be unique.
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: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
Network Infrastructure and Connectivity — This question tests Network Infrastructure and Connectivity — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: On R1, change the IPv4 mask to 255.255.255.252; on R2, assign IPv4 address 192.0.2.2/30 and IPv6 address 2001:db8:1::2/64; configure a default route on R2 pointing to 192.0.2.1. — R1's IPv4 mask was /28, but the correct mask for the link should be /30 to avoid overlapping subnets (192.0.2.0/28 includes both .1 and .14, but they are on the same link). R2 had no IPv6 address configured. Additionally, R1's EUI-64 address was valid but R2 needed a static IPv6 address. The solution: on R1, change the mask to 255.255.255.252; on R2, assign an IPv4 address with mask /30 and a static IPv6 address 2001:db8:1::2/64; also add a default route on R2 pointing to 192.0.2.1 for IPv4. After these changes, both routers can ping each other's IPv4 and IPv6 addresses.
What should I do if I get this 200-301 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 200-301 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You are connected to R1. The network has R1, R2, and a multilayer switch MLS1. Configure IPv4 and IPv6 addressing on R1's interfaces so that R1 can ping both R2 (198.51.100.2) and MLS1 (203.0.113.2) via IPv4. Additionally, configure IPv6 on G0/1 using EUI-64 with prefix 2001:db8:1::/64 and verify that R1 can ping the IPv6 address of MLS1 (2001:db8:1::2). The current configuration has incorrect subnet masks and missing IPv6 settings, causing reachability failures.
hard
✓ A.Change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/64.
B.Change the subnet mask on G0/0 to 255.255.255.0, change G0/1 to 255.255.255.0, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/64.
C.Change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the static address 2001:db8:1::1/64.
D.Change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/32.
Why A: The interfaces on R1 were configured with subnet masks that were not /30, which is required for these point-to-point links. With an incorrect mask, R1 does not consider the neighboring IPs (198.51.100.2 and 203.0.113.2) as directly connected, preventing ARP resolution and IPv4 reachability. Additionally, IPv6 was missing on G0/1. To fix, change the subnet mask on G0/0 to 255.255.255.252, change G0/1 to 255.255.255.252, then configure IPv6 on G0/1 with the EUI-64 address using the prefix 2001:db8:1::/64. After these changes, pings succeed.
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