CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
R1#show running-config | section interface interface GigabitEthernet0/0 ip address 198.51.100.1 255.255.255.0 no shutdown ! interface GigabitEthernet0/1 ip address 203.0.113.1 255.255.255.0 no shutdown ! interface GigabitEthernet0/2 no ip address shutdown ! R1#show ip interface brief Interface IP-Address OK? Method Status Protocol GigabitEthernet0/0 198.51.100.1 YES manual up up GigabitEthernet0/1 203.0.113.1 YES manual up up GigabitEthernet0/2 unassigned YES manual administratively down down R1#ping 198.51.100.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 198.51.100.2, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) R1#ping 203.0.113.2 Type escape sequence to abort. Sending 5, 100-byte ICMP Echos to 203.0.113.2, timeout is 2 seconds: ..... Success rate is 0 percent (0/5) R1#show ipv6 interface brief GigabitEthernet0/0 [administratively down/down] GigabitEthernet0/1 [administratively down/down] Note: R2 is on subnet 198.51.100.0/30, MLS1 is on subnet 203.0.113.0/30. Both R2 and MLS1 have correct IPs and are reachable via their respective subnets.
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.
⚠ Common exam trap
This question tests your understanding of subnet masks and their impact on Layer 3 reachability. A common trap is to focus only on IPv6 and forget that incorrect IPv4 subnet masks can prevent ARP resolution, even if IPv6 is configured correctly. Also, pay close attention to the exact requirements: EUI-64 and the correct prefix length.
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
✓
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.
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.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
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.
Why this is correct
This is correct because the /24 masks were causing R1 to believe R2 and MLS1 were on different subnets, preventing ARP resolution. Changing to /30 masks puts the interfaces on the correct point-to-point subnets. Configuring IPv6 with EUI-64 on G0/1 generates the interface ID from the MAC address, allowing R1 to ping MLS1's IPv6 address.
- ✗
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.
Why it's wrong here
This is incorrect because the /24 masks are the original problem; they do not provide the correct subnet boundaries for the point-to-point links. The IPv6 configuration is correct, but the IPv4 masks remain wrong, so pings will still fail.
- ✗
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.
Why it's wrong here
This is incorrect because the IPv6 address must be generated using EUI-64 as specified in the question. Using a static address may not match the expected address on MLS1, and the question explicitly requires EUI-64.
- ✗
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 it's wrong here
This is incorrect because the prefix length for the IPv6 address should be /64, not /32. Using a /32 prefix would place the interface on a much larger subnet, potentially causing routing issues and not matching MLS1's address.
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.
✓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.Correct answer▾
Why this is correct
This is correct because the /24 masks were causing R1 to believe R2 and MLS1 were on different subnets, preventing ARP resolution. Changing to /30 masks puts the interfaces on the correct point-to-point subnets. Configuring IPv6 with EUI-64 on G0/1 generates the interface ID from the MAC address, allowing R1 to ping MLS1's IPv6 address.
✗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.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that /24 masks are too large for point-to-point links and do not match the expected subnets for R2 and MLS1.
Why candidates choose this
Candidates might think that keeping the /24 mask is fine because they are focusing only on IPv6, or they may not realize that the /24 mask causes R1 to believe the remote addresses are on different subnets.
✗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.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the IPv6 address should be configured with the EUI-64 keyword, not a static address.
Why candidates choose this
Candidates might think that any IPv6 address in the same subnet will work, or they may be more comfortable with static addressing and overlook the EUI-64 requirement.
✗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.Wrong answer — click to see why▾
Why this is wrong here
The specific factual error is that the prefix length must be /64 as specified in the question; a /32 prefix is incorrect for this scenario.
Why candidates choose this
Candidates might confuse the prefix length with the network prefix or think that a shorter prefix is acceptable, not realizing that EUI-64 requires a /64 prefix.
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?”
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Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
ICMP
ICMP is a network-layer protocol used by network devices to send error messages and operational information about network connectivity.
Key term
EUI-64
EUI-64 is a method for creating a 64-bit interface identifier from a 48-bit MAC address, used to form an IPv6 link-local or stateless address auto-configuration (SLAAC) address.
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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.