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Good Morning,

As this is a homework question, I'm hoping for some guidance in the correct direction or a website. I was given an IPv6 address and I need to find the EUI portion of the address. And EUI is just the first 24 bits of a MAC address with the seventh bit flipped, followed by FFFE (16 bits), and the last 24 bits of the MAC address. Naturally, if you see FFFE in the address anywhere, it's an EUI address.

These are the IPv6 addresses:
2005:007D:4500:000C:0000:0010:11D8:6E71
2005:007D:4500:000C:0000:0002:4A5C:E1C9

The prefix is 2005:7D:4500:C::/64

FFFE isn't in either of them. Did I completly miss something regarding EUI addressing? I did reach out to the professor as it's an online course but I wanted to see if anyone here can identify if something is off with this or if I'm just compleltly missing something. I don't think 0000:0002:4A5C:E1C9 is a valid EUI.

Regards,
Ken

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Quick clarification and a practical checklist for readers who land here later.

's final post shows the assignment turned out to be a typo; was also correct that an IPv6 address does not have to contain a MAC (DHCP, privacy addresses, and manual assignment are common). To decide whether a suffix is EUI-derived and to recover a MAC when it is, follow these simple steps.

  • Make sure the network is a /64. EUI-derived interface IDs occupy the lower 64 bits.
  • Isolate the lower 64 bits (the interface ID) and look at the middle two bytes of that 8-octet field. If those two bytes equal 0xFFFE, the address is the usual EUI-48 → EUI-64 mapping. Remove the 0xFFFE bytes, then toggle the interface ID's first octet by XORing it with 0x02 to restore the original 48-bit MAC.
  • If 0xFFFE is not present, it is not the standard 48-bit-to-64-bit mapping. Possibilities: the interface ID came from a native 64-bit EUI (no FFFE insertion), it is a privacy/temporary address, or it was assigned by DHCPv6 or set manually. In those cases you cannot reliably recover a 48-bit MAC from the IPv6 suffix.

Practical checks: inspect the host to confirm the real MAC and address assignment (for example ip link / ip -6 addr on Linux or ipconfig /all on Windows), and check the neighbor table to map IPv6 to link-layer addresses. When an assignment or exercise looks inconsistent, double-check the instructor’s data — typos like the one in this thread happen.

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Member Avatar for rproffitt

Does not look to be a two way street. That is, it's a method we can use to automatically configure IPv6 host addresses. An IPv6 device will use the MAC address of its interface to generate a unique 64-bit interface ID. However, this doesn't mean that a DHCP assigned address will contain that MAC address.

Edited by rproffitt because: Spelling.
Member Avatar for rproffitt

I think we need to take a step back here. The IP address does not contain the MAC address. However there was a need to increase the MAC address pool for IPv6. "Guidelines for 48-Bit Global Identifier (EUI-48)" (PDF). IEEE Standards Association. IEEE. Retrieved 16 April 2015 gives a great overview of this.

So let's recap.

ARP for IPv4 to get the MAC.
NDP for IPv6 to get the new EUI.

Member Avatar for kes166

Thanks I'll check it out

Member Avatar for kes166

So the network ID is clearly:
2005:7D:4500:C::/64
It states this.

If we assume it's EUI-48, that would make them
0010:11D8:6E71
0002:4A5C:E1C9
Respectivly to the first post

Since it does not contain FFFE or FFFF, it is not an EUI-48 mapped to an EUI-64; therfore, the entire EUI-48 does not need to be the interface ID. I think that's where I was getting stuck on assuming my logic on this is correct. The interface ID's are:
0000:0010:11D8:6E71
0000:0002:4A5C:E1C9

At least this is the answer I'm going with :)

Thanks,
Ken

Member Avatar for kes166

Hey,

Just an update, there was a typo on the assignment.

The MAC address for computer A is 00-10-11-D8-6E-71
Computer B is 00-02-4A-5C-E1-C9

The put the MAC address on the assignment in the form of an IPv6 address :-/

They want me to make the rest of the IPv6 which is easy.

It suddenly makes sense.

Edited by kes166
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