What causes identical IP addresses with two SLAAC-configured computers?

A new computer that was connected to the network reported an error of an Identical IP address with another computer. Both computers were configured for SLAAC. Which of the following is causing the error?

Rogue DHCP server
Duplicate MAC addresses
Incorrect router advertisement Source Reference Answer
Wrong VLAN assignment

Community Votes

71%

29%

71% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

This question tests how SLAAC builds IPv6 host addresses from MAC addresses; the trap is confusing router advertisements with DHCP-style address assignment, when RA only supplies the network prefix.

In the CompTIA Network+ N10-008 exam, a duplicate IPv6 address error between two SLAAC computers is best explained by duplicate MAC addresses. Community discussion supports that SLAAC derives its EUI-64 interface identifier from the MAC address, making an incorrect router advertisement a common but incorrect choice.

The most common wrong answer is C, Incorrect router advertisement. Many learners assume a bad RA can assign identical IPv6 addresses, but SLAAC host bits come from the MAC address, so two devices with the same MAC will generate the same IPv6 address.

Community Discussion (6 comments)

a87d6a4 👍 7 Selected: C

In the context of SLAAC (Stateless Address Autoconfiguration), if two computers report an identical IP address error, the issue is most likely caused by an incorrect router advertisement. This happens because SLAAC relies on router advertisements to provide the network prefix. If the router sends incorrect or duplicate advertisements, it can lead to IP address conflicts as multiple devices may end up with the same address. A. Rogue DHCP server: A rogue DHCP server could cause issues with DHCP addresses, but SLAAC is not dependent on DHCP for IP assignment; it uses router advertisements. B. Duplicate MAC addresses: Duplicate MAC addresses are not related to IP address assignment issues but rather would cause other network problems, such as addressing conflicts at the link layer. D. Wrong VLAN assignment: Incorrect VLAN assignment might cause connectivity issues or misrouting of traffic but would not directly lead to IP address conflicts in the context of SLAAC. If I had to make second choice then I'd go with A.

kolab007 👍 1 Selected: B

When both computers are configured for SLAAC (Stateless Address Autoconfiguration), the IPv6 addresses are typically generated based on the MAC address of the network interface card (NIC). If two devices on the same network have duplicate MAC addresses, they will generate identical IPv6 addresses using SLAAC, leading to an IP address conflict.

dadba8c 👍 2

B. Duplicate MAC addresses "In SLAAC, the IPv6 address is typically generated based on the MAC address of the network interface. If two devices have the same MAC address, they will generate the same IPv6 address, leading to an IP address conflict. Duplicate MAC addresses are rare but can occur due to manufacturing errors or cloning of virtual machines. An Incorrect router advertisement could cause issues with network configuration but would not result in identical IP addresses being assigned by SLAAC."

veggietron9000 👍 1 Selected: B

With EUI-64 (Extended Unique Identifier-64), the calculation is consistent each time for a given MAC address. Here's a more detailed explanation: Consistent Algorithm: EUI-64 uses a standardized algorithm to convert a 48-bit MAC address into a 64-bit interface identifier for IPv6 addresses. Process: The algorithm takes the 48-bit MAC address. It inserts 'FFFE' in the middle (after the first 24 bits). It flips the 7th bit of the first byte (the Universal/Local bit). Reproducibility: This process will always produce the same 64-bit identifier for a given MAC address.

a737543 👍 3 Selected: C

In an IPv6 network using SLAAC, devices configure their IP addresses based on the prefix advertised by routers through Router Advertisement (RA) messages. If there is an issue with the RA messages, such as an incorrect or duplicate prefix being advertised, it can lead to multiple devices receiving the same IP address.

TrebleSmith 👍 2 Selected: B

B is going to be the best answer here. SLAAC uses MAC addresses to assign IPv6 addresses.

Comments & Corrections

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Expert Analysis

Why the Answer Is Correct

B is correct because SLAAC uses the interface identifier generated from the device's MAC address (EUI-64) to complete the IPv6 address. If two computers have identical MAC addresses, they will compute the same interface identifier and, when combined with the same advertised prefix, create the same IPv6 address, causing a conflict as described in the question.

Why the Other Options Are Wrong

A rogue DHCP server is not relevant because SLAAC is stateless and does not use DHCP for address assignment. An incorrect router advertisement may provide a wrong prefix or gateway, but it does not assign full host addresses—the host portion comes from the MAC. A wrong VLAN assignment would isolate traffic rather than create two identical addresses on the same network segment.

Community Comment Notes

Comments supporting C incorrectly state that a router advertisement can make multiple devices receive the same IP address, but RA only advertises network parameters. More useful comments note that SLAAC creates addresses from MAC addresses, so duplicate MACs cause duplicate IPv6 addresses, as correctly described by several respondents.

Official Reference

Exam Strategy

Remember that SLAAC is a host-autoconfiguration mechanism, not a server-allocation one. If the question reports identical IPv6 addresses, think about what is identical on the two hosts—their MAC addresses—not router-side configuration.

Related Analysis

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