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. Configure IPv4 and IPv6 addressing on R1's interfaces so that R1 can reach R2's loopback0 (192.0.2.1/32) and R2's IPv6 loopback0 (2001:db8:1::1/64). R1 has a misconfigured subnet mask on G0/0 and is missing its default gateway. Additionally, R1 has a duplicate IPv4 address on G0/1 that must be corrected. Use EUI-64 for R1's IPv6 link-local address on G0/0 and static IPv6 for the global unicast address on G0/1.
R1#show running-config | section interface
interface GigabitEthernet0/0
ip address 10.0.0.1 255.255.255.0
duplex auto
speed auto
!
interface GigabitEthernet0/1
ip address 10.0.0.5 255.255.255.252
duplex auto
speed auto
!
interface Loopback0
ip address 198.51.100.1 255.255.255.0
!
R1#show ip interface brief
Interface IP-Address OK? Method Status Protocol
GigabitEthernet0/0 10.0.0.1 YES manual up up
GigabitEthernet0/1 10.0.0.5 YES manual up up
Loopback0 198.51.100.1 YES manual up up
R1#show ipv6 interface brief
GigabitEthernet0/0 [unassigned]
GigabitEthernet0/1 [unassigned]
Loopback0 [unassigned]
R1#show ip route
Codes: C - connected, S - static, I - IGRP, 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
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.0.0.0/24 is directly connected, GigabitEthernet0/0
198.51.100.0/24 is subnetted, 1 subnets
C 198.51.100.0/24 is directly connected, Loopback0
R1#ping 10.0.0.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.2, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5)
A
Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
This option correctly fixes all issues: the subnet mask on G0/0 is changed to /30 to match the connected network, a default route via 10.0.0.2 provides a gateway to reach R2's loopbacks, G0/1 gets a unique IP (192.0.2.2/30) to resolve the duplicate, IPv6 is enabled on G0/0 with EUI-64 for link-local, and G0/1 gets a static global unicast address (2001:db8:2::1/64) as required.
B
Change G0/0 mask to /24, add default route via 10.0.0.1, change G0/1 IP to 10.0.0.6/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:1::1/64 to G0/1
Why wrong: This is incorrect because changing G0/0 mask to /24 does not match the /30 subnet used by R2, so R1 cannot communicate with R2's G0/0. The default route via 10.0.0.1 is not the correct next-hop (should be 10.0.0.2). Changing G0/1 to 10.0.0.6/30 still conflicts with R2's G0/1 (10.0.0.5/30) because 10.0.0.6 is in the same subnet. Assigning 2001:db8:1::1/64 to G0/1 duplicates R2's loopback address.
C
Change G0/0 mask to /30, add default route via 10.0.0.1, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
Why wrong: This is incorrect because the default route should point to 10.0.0.2, not 10.0.0.1. R2's G0/0 IP is 10.0.0.2, so the next-hop must be 10.0.0.2. Using 10.0.0.1 as the gateway would send traffic to an incorrect or non-existent neighbor.
D
Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 10.0.0.6/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
Why wrong: This is incorrect because changing G0/1 to 10.0.0.6/30 still conflicts with R2's G0/1 (10.0.0.5/30). Both addresses are in the same /30 subnet (10.0.0.4/30), causing a duplicate IP issue. The correct action is to assign an IP from a different subnet, such as 192.0.2.2/30.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
R1 cannot reach R2 because G0/0 has a wrong subnet mask (/24 instead of /30) and no default gateway. Also, G0/1 has a duplicate IPv4 address (10.0.0.5/30 conflicts with R2's G0/1). To fix: change G0/0 mask to /30, add a default route via 10.0.0.2, assign a unique IP to G0/1 (e.g., 192.0.2.2/30), enable IPv6 on G0/0 with EUI-64link-local, and assign a static global unicast address to G0/1 (2001:db8:2::1/64).
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.
✓
Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
Why this is correct
This option correctly fixes all issues: the subnet mask on G0/0 is changed to /30 to match the connected network, a default route via 10.0.0.2 provides a gateway to reach R2's loopbacks, G0/1 gets a unique IP (192.0.2.2/30) to resolve the duplicate, IPv6 is enabled on G0/0 with EUI-64 for link-local, and G0/1 gets a static global unicast address (2001:db8:2::1/64) as required.
Change G0/0 mask to /24, add default route via 10.0.0.1, change G0/1 IP to 10.0.0.6/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:1::1/64 to G0/1
Why it's wrong here
This is incorrect because changing G0/0 mask to /24 does not match the /30 subnet used by R2, so R1 cannot communicate with R2's G0/0. The default route via 10.0.0.1 is not the correct next-hop (should be 10.0.0.2). Changing G0/1 to 10.0.0.6/30 still conflicts with R2's G0/1 (10.0.0.5/30) because 10.0.0.6 is in the same subnet. Assigning 2001:db8:1::1/64 to G0/1 duplicates R2's loopback address.
✗
Change G0/0 mask to /30, add default route via 10.0.0.1, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
Why it's wrong here
This is incorrect because the default route should point to 10.0.0.2, not 10.0.0.1. R2's G0/0 IP is 10.0.0.2, so the next-hop must be 10.0.0.2. Using 10.0.0.1 as the gateway would send traffic to an incorrect or non-existent neighbor.
✗
Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 10.0.0.6/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1
Why it's wrong here
This is incorrect because changing G0/1 to 10.0.0.6/30 still conflicts with R2's G0/1 (10.0.0.5/30). Both addresses are in the same /30 subnet (10.0.0.4/30), causing a duplicate IP issue. The correct action is to assign an IP from a different subnet, such as 192.0.2.2/30.
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 G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1Correct answer▾
Why this is correct
This option correctly fixes all issues: the subnet mask on G0/0 is changed to /30 to match the connected network, a default route via 10.0.0.2 provides a gateway to reach R2's loopbacks, G0/1 gets a unique IP (192.0.2.2/30) to resolve the duplicate, IPv6 is enabled on G0/0 with EUI-64 for link-local, and G0/1 gets a static global unicast address (2001:db8:2::1/64) as required.
✗Change G0/0 mask to /24, add default route via 10.0.0.1, change G0/1 IP to 10.0.0.6/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:1::1/64 to G0/1Wrong answer — click to see why▾
Why this is wrong here
The subnet mask on G0/0 must match the connected network (/30), not /24. The default gateway should point to the neighbor's IP (10.0.0.2). The new G0/1 IP must be in a different subnet to avoid duplication. The IPv6 global unicast address on G0/1 must be unique and not conflict with R2's loopback.
Why candidates choose this
Candidates might think /24 is a common mask and that 10.0.0.1 is a typical gateway. They may also mistakenly believe that 10.0.0.6/30 is different from 10.0.0.5/30, not realizing they are in the same subnet. Using 2001:db8:1::1/64 might seem like a valid global unicast address but it duplicates R2's loopback.
✗Change G0/0 mask to /30, add default route via 10.0.0.1, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1Wrong answer — click to see why▾
Why this is wrong here
The default gateway must be the IP address of the directly connected neighbor (R2's G0/0), which is 10.0.0.2, not 10.0.0.1.
Why candidates choose this
Candidates often assume the default gateway is the first usable IP in the subnet (10.0.0.1) without verifying the actual neighbor's IP. They may also confuse the router's own IP with the gateway.
✗Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 10.0.0.6/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1Wrong answer — click to see why▾
Why this is wrong here
The IP address 10.0.0.6/30 is in the same subnet as R2's G0/1 (10.0.0.5/30), so it does not resolve the duplicate address conflict. A different subnet must be used.
Why candidates choose this
Candidates might think that 10.0.0.6 is different from 10.0.0.5 and therefore not a duplicate, but they overlook that both are in the same /30 subnet. They may also assume that using a different host ID within the same subnet is sufficient.
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: Change G0/0 mask to /30, add default route via 10.0.0.2, change G0/1 IP to 192.0.2.2/30, enable IPv6 on G0/0 with EUI-64 link-local, assign 2001:db8:2::1/64 to G0/1 — R1 cannot reach R2 because G0/0 has a wrong subnet mask (/24 instead of /30) and no default gateway. Also, G0/1 has a duplicate IPv4 address (10.0.0.5/30 conflicts with R2's G0/1). To fix: change G0/0 mask to /30, add a default route via 10.0.0.2, assign a unique IP to G0/1 (e.g., 192.0.2.2/30), enable IPv6 on G0/0 with EUI-64 link-local, and assign a static global unicast address to G0/1 (2001:db8:2::1/64).
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.
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