CCNA Network Infrastructure and Connectivity Practice Question
Network Topology
You are connected to R1. Configure IPv4 and IPv6 addressing on R1's interfaces and verify reachability to R2. The current configuration has a wrong subnet mask on G0/0, missing default gateway for IPv4, and R1's IPv6 address is configured using EUI-64 while R2 uses a static IPv6 address. Fix these issues so that R1 can ping both R2's IPv4 and IPv6 addresses.
⚠ Common exam trap
Be careful not to confuse the default gateway with an arbitrary IP; it must be the next-hop router's interface IP on the same subnet. Also, remember that EUI-64 generates a unique interface ID from the MAC, which may not match a statically configured peer address—both sides must use consistent addressing methods.
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 R1 G0/0 subnet mask to /30, add a default route via 192.0.2.2, and configure a static IPv6 address 2001:db8:1::1/64 on G0/0.
The problem had three issues: (1) R1's G0/0 subnet mask was /24 (255.255.255.0) but R2's G0/0 was /30 (255.255.255.252), causing an IP subnet mismatch. (2) R1 lacked a default gateway for IPv4; the static route pointed to 192.0.2.254 which is not reachable. (3) R1's IPv6 EUI-64 configuration on G0/0 generates an interface ID from the MAC, but R2 expects a static address 2001:db8:1::2/64, so R1 must use a static IPv6 address on the same subnet. The fix: change R1's G0/0 mask to /30, add a default route via R2's G0/0 IP (192.0.2.2), and configure a static IPv6 address (e.g., 2001:db8:1::1/64) on R1's G0/0.
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 R1 G0/0 subnet mask to /30, add a default route via 192.0.2.2, and configure a static IPv6 address 2001:db8:1::1/64 on G0/0.
Why this is correct
This corrects all three issues: the mask matches R2's /30, the default route points to R2's G0/0 IP (192.0.2.2), and the static IPv6 address places R1 on the same subnet as R2's static address 2001:db8:1::2/64.
- ✗
Change R1 G0/0 subnet mask to /30, add a default route via 192.0.2.254, and keep the EUI-64 IPv6 address on G0/0.
Why it's wrong here
This is incorrect because the default route points to 192.0.2.254, which is not R2's G0/0 IP (192.0.2.2), so IPv4 traffic will not be forwarded. Also, EUI-64 generates an interface ID from the MAC, which will not match the subnet expected by R2's static address 2001:db8:1::2/64.
- ✗
Change R1 G0/0 subnet mask to /24, add a default route via 192.0.2.2, and configure a static IPv6 address 2001:db8:1::1/64 on G0/0.
Why it's wrong here
This is incorrect because the subnet mask on R1's G0/0 remains /24, which does not match R2's /30. R1 will consider the subnet to be 192.0.2.0/24, while R2 uses 192.0.2.0/30, causing a mismatch that prevents direct communication.
- ✗
Change R1 G0/0 subnet mask to /30, add a default route via 192.0.2.2, and keep the EUI-64 IPv6 address on G0/0.
Why it's wrong here
This is incorrect because while the IPv4 issues are fixed, the IPv6 address generated by EUI-64 will not match the subnet expected by R2's static address 2001:db8:1::2/64. EUI-64 creates an interface ID based on the MAC, which is unpredictable and unlikely to be in the same subnet as R2's static 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 R1 G0/0 subnet mask to /30, add a default route via 192.0.2.2, and configure a static IPv6 address 2001:db8:1::1/64 on G0/0.Correct answer▾
Why this is correct
This corrects all three issues: the mask matches R2's /30, the default route points to R2's G0/0 IP (192.0.2.2), and the static IPv6 address places R1 on the same subnet as R2's static address 2001:db8:1::2/64.
✗Change R1 G0/0 subnet mask to /30, add a default route via 192.0.2.254, and keep the EUI-64 IPv6 address on G0/0.Wrong answer — click to see why▾
Why this is wrong here
The default gateway must be R2's directly connected interface IP (192.0.2.2), not 192.0.2.254. EUI-64 cannot be used if the peer expects a specific static address on the same subnet.
Why candidates choose this
Candidates may think the original default route (192.0.2.254) is correct or that EUI-64 is always acceptable for IPv6 connectivity.
✗Change R1 G0/0 subnet mask to /24, add a default route via 192.0.2.2, and configure a static IPv6 address 2001:db8:1::1/64 on G0/0.Wrong answer — click to see why▾
Why this is wrong here
The subnet mask must be consistent on both ends of the link. A /24 mask on one side and /30 on the other creates overlapping subnets and routing issues.
Why candidates choose this
Candidates might think the mask only needs to be consistent within the same network or that a /24 is a common default mask, overlooking the specific /30 used by R2.
✗Change R1 G0/0 subnet mask to /30, add a default route via 192.0.2.2, and keep the EUI-64 IPv6 address on G0/0.Wrong answer — click to see why▾
Why this is wrong here
EUI-64 does not guarantee that the resulting IPv6 address will be on the same subnet as a statically configured peer address. For direct connectivity, both routers must have addresses in the same subnet.
Why candidates choose this
Candidates may assume EUI-64 is always compatible or that IPv6 neighbor discovery will automatically resolve the mismatch.
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 route
A default route is a catch-all routing entry that tells a network device where to send packets when no specific route matches the destination address.
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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.