CCNA Network Infrastructure and Connectivity Practice Question
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.
⚠ Common exam trap
Watch for subnet mask mismatches on point-to-point links; they break routing. Also, remember that IPv6 unicast-routing is disabled by default and must be enabled. EUI-64 requires the 'eui-64' keyword and generates an address based on MAC, not a static ::1.
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 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.
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?”
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Key term
IPv6
IPv6 is the most recent version of the Internet Protocol, designed to replace IPv4 by providing a vastly larger number of unique addresses and improved network features.
Key term
IPv6 address
An IPv6 address is a 128-bit numeric label used to identify a device on an Internet Protocol network, designed to replace IPv4 due to the exhaustion of available addresses.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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