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CCNA Practice Question: A network administrator is troubleshooting an…

This 200-301 practice question tests your understanding of 200-301 exam topics. 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.

Exhibit

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 00:1a:2b:3c:4d:5e brd ff:ff:ff:ff:ff:ff
    inet6 fe80::21a:2bff:fe3c:4d5e/64 scope link
       valid_lft forever preferred_lft forever
    inet6 2001:db8:acad:1::21a:2bff:fe3c:4d5e/64 scope global dynamic
       valid_lft 2592000sec preferred_lft 604800sec
    inet6 2001:db8:acad:1::1/64 scope global
       valid_lft forever preferred_lft forever

A network administrator is troubleshooting an IPv6 connectivity issue on a newly deployed host. The host is configured to obtain an IPv6 address via SLAAC, but it cannot reach the default gateway. The administrator runs 'ip addr show dev eth0' on the host and sees the following output. What is the most likely cause of the problem?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

  • Clue: "most likely"

    Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

Question 1hardmultiple choice
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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

✓

The host is missing a default route; the 'ip addr show' output does not show any routes, so the host cannot reach the default gateway.

The host has a manually configured static IPv6 address (2001:db8:acad:1::1) in addition to the SLAAC-derived address. This static address is on the same subnet as the SLAAC address, so it does not cause a duplicate address conflict. However, the host still has the correct default gateway from the Router Advertisement (RA) as indicated by the 'dynamic' address. The problem is that the host does not have a default route; the 'ip addr show' output shows only addresses, not routes. The administrator should verify the routing table with 'ip -6 route show' to see if a default route via the RA-learned gateway exists. Without a default route, the host cannot send packets off the local link.

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.

  • ✗

    The host has a duplicate IPv6 address conflict because the manually configured address overlaps with the SLAAC address.

    Why it's wrong here

    The manually configured address and the SLAAC address are different; there is no overlap. The duplicate address detection (DAD) would have flagged an issue if they were the same.

  • ✓

    The host is missing a default route; the 'ip addr show' output does not show any routes, so the host cannot reach the default gateway.

    Why this is correct

    The 'ip addr show' command only displays addresses, not routes. The host likely received a default gateway via RA, but the routing table must be checked separately. Without a default route, off-link communication fails.

    Clue confirmation

    The clue word "most likely" in the question point toward this answer.

    Related concept

    CIDR notation defines the prefix length.

  • ✗

    The host's SLAAC address is not using EUI-64 format, which prevents it from communicating with the router.

    Why it's wrong here

    The SLAAC address shown (2001:db8:acad:1::21a:2bff:fe3c:4d5e) does use EUI-64 format (the ff:fe in the middle). EUI-64 is not a requirement for SLAAC; it is just one method.

  • ✗

    The host has a manually configured static IPv6 address that overrides the SLAAC-learned default gateway.

    Why it's wrong here

    Manually configuring a static IPv6 address does not override the default gateway learned via RA. The host can have multiple addresses and still use the RA-learned default route.

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.

✓The host is missing a default route; the 'ip addr show' output does not show any routes, so the host cannot reach the default gateway.Correct answer▾

Why this is correct

The 'ip addr show' command only displays addresses, not routes. The host likely received a default gateway via RA, but the routing table must be checked separately. Without a default route, off-link communication fails.

✗The host has a duplicate IPv6 address conflict because the manually configured address overlaps with the SLAAC address.Wrong answer — click to see why▾

Why this is wrong here

The addresses are different, so no conflict exists.

✗The host's SLAAC address is not using EUI-64 format, which prevents it from communicating with the router.Wrong answer — click to see why▾

Why this is wrong here

EUI-64 is used here, but even if it weren't, that would not cause connectivity failure.

✗The host has a manually configured static IPv6 address that overrides the SLAAC-learned default gateway.Wrong answer — click to see why▾

Why this is wrong here

Static address configuration does not remove the RA-learned default route.

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.

Trap categories for this question

  • Command / output trap

    The SLAAC address shown (2001:db8:acad:1::21a:2bff:fe3c:4d5e) does use EUI-64 format (the ff:fe in the middle). EUI-64 is not a requirement for SLAAC; it is just one method.

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.

What to study next

Got this wrong? Here's your next step.

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.

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FAQ

Questions learners often ask

What does this 200-301 question test?

CIDR notation defines the prefix length.

What is the correct answer to this question?

The correct answer is: The host is missing a default route; the 'ip addr show' output does not show any routes, so the host cannot reach the default gateway. — The host has a manually configured static IPv6 address (2001:db8:acad:1::1) in addition to the SLAAC-derived address. This static address is on the same subnet as the SLAAC address, so it does not cause a duplicate address conflict. However, the host still has the correct default gateway from the Router Advertisement (RA) as indicated by the 'dynamic' address. The problem is that the host does not have a default route; the 'ip addr show' output shows only addresses, not routes. The administrator should verify the routing table with 'ip -6 route show' to see if a default route via the RA-learned gateway exists. Without a default route, the host cannot send packets off the local link.

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.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

CIDR notation defines the prefix length.

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