Correct Subnet Masks and Configure Dual-Stack IPv4/IPv6 on Routers
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.
Quick Answer
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.
⚠ Common exam trap
Watch out for subnet mask mismatches and duplicate IPv6 addresses. Always use /30 for point-to-point links and ensure each router has a unique IPv6 address on the same link.
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
✓
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.
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?”
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.
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Same concept, more angles
1 more way this is tested on 200-301
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.
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.