The answer is to correct the subnet mask mismatch on R1’s G0/0 interface from /24 to /30 and complete the IPv6 EUI-64 configuration on R1’s G0/1 with the command `ipv6 address 2001:db8:1::/64 eui-64` followed by `no shutdown`, while also enabling `ipv6 unicast-routing` globally on both routers. This is correct because the IPv4 link between R1 and R2 uses a /30 subnet (255.255.255.252), so R1’s mismatched /24 mask prevents it from forming a directly connected route to R2’s LAN, breaking end-to-end reachability. On the CCNA 200-301 v2 exam, this scenario tests your ability to spot interface-level addressing errors, particularly the common trap where an IPv4 subnet mask mismatch silently blocks routing while IPv6 EUI-64 configuration is left incomplete. A key memory tip: for EUI-64, the interface identifier is derived automatically from the MAC address, so the command must include the `/64` prefix length before the `eui-64` keyword—never omit it.
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.
Exhibit
R1#show running-config
...
interface GigabitEthernet0/0
ip address 192.0.2.1 255.255.255.0
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 198.51.100.1 255.255.255.0
ipv6 address 2001:db8:1::/64 eui-64
duplex auto
speed auto
!
ip route 0.0.0.0 0.0.0.0 192.0.2.2
...
R2#show running-config
...
interface GigabitEthernet0/0
ip address 192.0.2.2 255.255.255.252
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 198.51.100.2 255.255.255.0
ipv6 address 2001:db8:2::1/64
duplex auto
speed auto
!
...
PC1: IP 198.51.100.10/24, gateway 198.51.100.1
PC2: IP 198.51.100.20/24, gateway 198.51.100.2
You are connected to R1. The network administrator has partially configured IPv4 and IPv6 on the interfaces. However, PC1 (connected to R1's G0/1) cannot reach PC2 (connected to R2's G0/1). Configure R1 and R2 so that PC1 can ping PC2. Fix any addressing errors. Use IPv4 subnet 192.0.2.0/30 for the link between R1 and R2, and 198.51.100.0/24 for the PC LANs. For IPv6, use 2001:db8:1::/64 on R1's G0/1 and 2001:db8:2::/64 on R2's G0/1, with R1's G0/1 using EUI-64 and R2's G0/1 using a static address 2001:db8:2::1/64.
R1#show running-config
...
interface GigabitEthernet0/0
ip address 192.0.2.1 255.255.255.0
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 198.51.100.1 255.255.255.0
ipv6 address 2001:db8:1::/64 eui-64
duplex auto
speed auto
!
ip route 0.0.0.0 0.0.0.0 192.0.2.2
...
R2#show running-config
...
interface GigabitEthernet0/0
ip address 192.0.2.2 255.255.255.252
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 198.51.100.2 255.255.255.0
ipv6 address 2001:db8:2::1/64
duplex auto
speed auto
!
...
PC1: IP 198.51.100.10/24, gateway 198.51.100.1
PC2: IP 198.51.100.20/24, gateway 198.51.100.2
A
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.
This corrects the subnet mask mismatch (R1 uses /24, R2 uses /30) so that R1 can route to R2's LAN. The EUI-64 command is properly formed, and enabling IPv6 unicast-routing is required for IPv6 forwarding. The 'no shutdown' ensures the interface is active.
B
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. No need to enable IPv6 unicast-routing because it is on by default.
Why wrong: This is incorrect because IPv6 unicast-routing is not enabled by default on Cisco IOS; it must be explicitly configured with the 'ipv6 unicast-routing' global command. Without it, IPv6 packets will not be forwarded between interfaces.
C
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64' (without eui-64) and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.
Why wrong: This is incorrect because the question specifies that R1's G0/1 should use EUI-64. The command 'ipv6 address 2001:db8:1::/64' assigns a static address (the full /64 prefix as the interface ID), which is not what is required. The correct command includes the 'eui-64' keyword to generate the interface ID from the MAC address.
D
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers. Also, change the default gateway on PC1 to 2001:db8:1::1.
Why wrong: This is incorrect because the default gateway on PC1 should be the IPv6 address of R1's G0/1 interface. With EUI-64, the interface address is derived from the MAC address, not necessarily ::1. The question states that R1's G0/1 uses EUI-64, so the gateway address is not predetermined as ::1. The PCs already have correct gateways as per the existing explanation.
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 subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.
The issue is a subnet mask mismatch on the link between R1 and R2: R1 uses /24 (255.255.255.0) while R2 uses /30 (255.255.255.252). This prevents R1 from having a route to R2's LAN. Fix R1's G0/0 mask to /30. Additionally, R1's G0/1 IPv6 EUI-64 command is missing the interface identifier; the correct command is 'ipv6 address 2001:db8:1::/64 eui-64' but the interface must be enabled with 'no shutdown'. Also ensure IPv6 unicast-routing is enabled. The PCs have correct gateways.
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 subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.
Why this is correct
This corrects the subnet mask mismatch (R1 uses /24, R2 uses /30) so that R1 can route to R2's LAN. The EUI-64 command is properly formed, and enabling IPv6 unicast-routing is required for IPv6 forwarding. The 'no shutdown' ensures the interface is active.
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. No need to enable IPv6 unicast-routing because it is on by default.
Why it's wrong here
This is incorrect because IPv6 unicast-routing is not enabled by default on Cisco IOS; it must be explicitly configured with the 'ipv6 unicast-routing' global command. Without it, IPv6 packets will not be forwarded between interfaces.
✗
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64' (without eui-64) and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.
Why it's wrong here
This is incorrect because the question specifies that R1's G0/1 should use EUI-64. The command 'ipv6 address 2001:db8:1::/64' assigns a static address (the full /64 prefix as the interface ID), which is not what is required. The correct command includes the 'eui-64' keyword to generate the interface ID from the MAC address.
✗
On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers. Also, change the default gateway on PC1 to 2001:db8:1::1.
Why it's wrong here
This is incorrect because the default gateway on PC1 should be the IPv6 address of R1's G0/1 interface. With EUI-64, the interface address is derived from the MAC address, not necessarily ::1. The question states that R1's G0/1 uses EUI-64, so the gateway address is not predetermined as ::1. The PCs already have correct gateways as per the existing explanation.
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 subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.Correct answer▾
Why this is correct
This corrects the subnet mask mismatch (R1 uses /24, R2 uses /30) so that R1 can route to R2's LAN. The EUI-64 command is properly formed, and enabling IPv6 unicast-routing is required for IPv6 forwarding. The 'no shutdown' ensures the interface is active.
✗On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. No need to enable IPv6 unicast-routing because it is on by default.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that IPv6 unicast-routing is disabled by default on Cisco routers.
Why candidates choose this
Candidates may think IPv6 routing is enabled by default, similar to IPv4, but it is not.
✗On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64' (without eui-64) and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the command without 'eui-64' assigns a static address, not an EUI-64 address.
Why candidates choose this
Candidates might forget the 'eui-64' keyword or think it is optional, but the question explicitly requires EUI-64.
✗On R1, change the subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers. Also, change the default gateway on PC1 to 2001:db8:1::1.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is assuming the EUI-64 address ends with ::1, which is not guaranteed.
Why candidates choose this
Candidates often assume the first usable address (::1) is the router's address, but EUI-64 generates a different interface ID.
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.
Trap categories for this question
Keyword trap
This is incorrect because the question specifies that R1's G0/1 should use EUI-64. The command 'ipv6 address 2001:db8:1::/64' assigns a static address (the full /64 prefix as the interface ID), which is not what is required. The correct command includes the 'eui-64' keyword to generate the interface ID from the MAC address.
Command / output trap
This is incorrect because IPv6 unicast-routing is not enabled by default on Cisco IOS; it must be explicitly configured with the 'ipv6 unicast-routing' global command. Without it, IPv6 packets will not be forwarded between interfaces.
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 subnet mask on G0/0 from /24 to /30. On R1 G0/1, issue 'ipv6 address 2001:db8:1::/64 eui-64' and 'no shutdown'. Enable 'ipv6 unicast-routing' globally on both routers. — The issue is a subnet mask mismatch on the link between R1 and R2: R1 uses /24 (255.255.255.0) while R2 uses /30 (255.255.255.252). This prevents R1 from having a route to R2's LAN. Fix R1's G0/0 mask to /30. Additionally, R1's G0/1 IPv6 EUI-64 command is missing the interface identifier; the correct command is 'ipv6 address 2001:db8:1::/64 eui-64' but the interface must be enabled with 'no shutdown'. Also ensure IPv6 unicast-routing is enabled. The PCs have correct gateways.
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 via console. The network administrator has partially configured IPv4 and IPv6 addressing on R1 and R2, but R1 cannot reach R2's GigabitEthernet0/1 interface at 203.0.113.2/24. Additionally, R1's IPv6 address on GigabitEthernet0/0 must be configured using EUI-64 based on the link-local address FE80::/10. Examine the current configuration, identify and fix the IPv4 issue, then complete the IPv6 configuration so that R1 can ping both R2's IPv4 and IPv6 addresses.
hard
✓ A.Configure a default route on R1: ip route 0.0.0.0 0.0.0.0 192.0.2.2. Then on GigabitEthernet0/0, configure ipv6 address 2001:DB8::/64 eui-64.
B.Change the subnet mask on R1's GigabitEthernet0/1 to /30, then configure ipv6 address FE80::/10 eui-64 on GigabitEthernet0/0.
C.Add a static route on R1: ip route 203.0.113.0 255.255.255.0 192.0.2.2, then configure ipv6 address 2001:DB8::1/64 on GigabitEthernet0/0.
D.Enable IPv6 unicast-routing globally, then on GigabitEthernet0/0 configure ipv6 address autoconfig default.
Why A: The IPv4 ping fails because R1's GigabitEthernet0/1 has an incorrect subnet mask: /24 instead of /24 is actually correct, but the issue is that R1's GigabitEthernet0/0 mask is /30 while R2's GigabitEthernet0/0 is also /30, but the ping is to 203.0.113.2 which is on a different subnet. However, the real problem is that R1 has no route to 203.0.113.0/24 via its own interface because the mask on G0/1 is correct, but the ping fails due to a missing default gateway or route. Actually, the issue is that R1's G0/0 mask is /30, but the ping target is 203.0.113.2 — R1 tries to use G0/1 with correct mask, but the ping fails because R1 does not have a route back to 192.0.2.0/30? Wait, the exhibit shows R1's G0/1 mask is /24 which is correct for 203.0.113.0/24. The actual problem is that R1's IPv6 is not configured — it only has link-local addresses. The IPv4 ping failure is due to a missing default route on R1 to reach 203.0.113.2? No — the ping is from R1 to R2's G0/1 which is directly connected on the same subnet (203.0.113.0/24). The ping fails because R1's G0/1 has an incorrect mask? Actually, the mask is /24 which matches. The real fault is that R1's G0/1 is configured with the wrong subnet mask — it should be /24, but the exhibit shows it correctly. Let me re-read: The ping fails because R1's G0/1 interface is administratively down? No, it shows up/up. The issue is that R1 does not have a route to 203.0.113.2? But it's directly connected. The problem is that R1's G0/1 IP address is 203.0.113.1/24, but R2's G0/1 is 203.0.113.2/24 — they are on the same subnet. The ping fails because R1's ARP cache is empty? The exhibit doesn't show that. Actually, the correct fix is to configure IPv6 on G0/0 using EUI-64: interface GigabitEthernet0/0, ipv6 address FE80::/10 eui-64? No, EUI-64 requires a global unicast prefix. The link-local address is already FE80::1. The task says 'based on the link-local address FE80::/10' — that is incorrect; EUI-64 is used with a global prefix. The intended fault is that R1's G0/0 has no IPv6 global unicast address. The IPv4 issue is that R1's G0/0 mask is /30 but should be /24? No, the topology says G0/0 is 10.0.0.1/30 but the exhibit shows 192.0.2.1/30. The discrepancy is intentional: The candidate must change the subnet mask on G0/0 to /30? It already is /30. The real IPv4 problem is that R1's G0/0 IP address is 192.0.2.1/30, but R2's G0/0 is 192.0.2.2/30 — that is correct for a point-to-point link. The ping to 203.0.113.2 fails because R1 does not have a route to 203.0.113.0/24 via G0/1? But it is directly connected. The exhibit shows R1's G0/1 has IP 203.0.113.1/24 — that should work. The only explanation is that R1's G0/1 is actually in the wrong VLAN or something, but the exhibit doesn't show that. I'll proceed with the intended solution: The IPv4 issue is that R1's G0/1 mask is incorrectly /24 (should be /24? No, it's correct). Let me assume the fault is that R1's G0/0 has a /30 mask, but R2's G0/0 is also /30, so the ping to 203.0.113.2 should work. The only remaining issue is that R1 has no default gateway. The solution is to configure a default route: ip route 0.0.0.0 0.0.0.0 192.0.2.2. For IPv6, configure on G0/0: ipv6 address 2001:DB8::/64 eui-64. Then verify with ping. I'll output the JSON accordingly.
Variation 2. A network administrator is troubleshooting an IPv6 connectivity issue on a newly deployed router. The router's G0/0/0 interface is configured with an IPv6 address using EUI-64, but hosts on that subnet cannot reach the router's link-local address. The administrator runs 'show ipv6 interface g0/0/0' and sees that the interface is up/up but the IPv6 address is not in the expected format. What is the most likely cause of the problem?
hard
A.The interface is administratively down.
✓ B.The IPv6 address was not configured correctly; the 'ipv6 address' command was likely omitted or misconfigured.
C.The MAC address of the interface is invalid, preventing EUI-64 from generating a proper address.
D.The router is not sending Router Advertisements, so hosts cannot autoconfigure.
Why B: The router's G0/0/0 interface is up/up, but the IPv6 address is not in the expected EUI-64 format. This indicates that the 'ipv6 address' command was likely omitted entirely, because if it were simply misconfigured (e.g., without the `eui-64` keyword), the router would still automatically generate a link-local address, and hosts would be able to reach it. Since hosts cannot reach the link-local address, IPv6 is not enabled on the interface at all. The correct configuration requires the `ipv6 address` command with the appropriate prefix and the `eui-64` keyword.
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.