IPv6 EUI-64 explained

Rene, So when manually configuring an IPv6 say for link local, it’s not just stuffing the MAC address with FFFE in the middle - you have to pay attention to inverting the 7th bit in the 1st 2 hexadecimal numbers correct? I have seen some videos where there is no mention of inverting bit 7. Thanks

Hi Itai,

Oops…the FFFF is a typo indeed, that should be 1111. Just fixed it.

When you use EUI-64 to automatically use the MAC address as input for the “host” part then the 7th bit will be inverted, this is all done automatically though.

If you make up some IPv6 addresses of your own then you can pick whatever addresses you like…


Hi Rene, Do you have an example of an interface with EUI-64 and without EUI-64? I understand that EUI-64 modify the Link Local and Global Unicast.

What happen if I want to configure an Unique Local?

Hi Diana,

Here’s an example:

R1#show ipv6 interface GigabitEthernet 0/1
GigabitEthernet0/1 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::F816:3EFF:FEE6:7777

That’s the IPv6 link-local address that uses EUI-64. You can configure it manually if you want:

R1(config)#int gi0/1                                                      
R1(config-if)#ipv6 address FE80:1111:1111:1111:ABCD:ABCD:ABCD:ABCD link-local
R1#show ipv6 interface GigabitEthernet 0/1 | include FE80
  IPv6 is enabled, link-local address is FE80:1111:1111:1111:ABCD:ABCD:ABCD:ABCD

Here’s a global unicast address with EUI-64:

R1(config-if)#ipv6 address 2001:1111:1111:1111::/64 eui-64

R1#show ipv6 interface GigabitEthernet 0/1 | include 2001:
    2001:1111:1111:1111:F816:3EFF:FEE6:7777, subnet is 2001:1111:1111:1111::/64

If you don’t want to use EUI-64 then you can configure global unicast addresses yourself:

R1(config-if)#ipv6 address 2001:1111:1111:1111:ABCD:ABCD:ABCD:ABCD/64

R1#show ipv6 interface GigabitEthernet 0/1 | include ABCD 
    2001:1111:1111:1111:ABCD:ABCD:ABCD:ABCD, subnet is 2001:1111:1111:1111::/64 

That’s it. Hope this helps.


Rene - If I’m getting my head wrapped around this correct then the following grid maps all possible bit combinations at the 7th as discussed:

0 0 0000 0010 2 2
1 1 0001 0011 3 3
2 2 0010 0000 0 0
3 3 0011 0001 1 1
4 4 0100 0110 6 6
5 5 0101 0111 7 7
6 6 0110 0100 4 4
7 7 0111 0101 5 5
8 8 1000 1010 A 10
9 9 1001 1011 B 11
A 10 1010 1000 8 8
B 11 1011 1001 9 9
C 12 1100 1110 E 14
D 13 1101 1111 F 15
E 14 1110 1100 C 12
F 15 1111 1101 D 13

If that all holds true then the underlying form is +2/+2/-2/-2 transitioning on every fourth place (0,4,8,12):

0 0 +2 0 2 2
1 1 +2 3 3
2 2 -2 0 0
3 3 -2 1 1
4 4 +2 4 6 6
5 5 +2 7 7
6 6 -2 4 4
7 7 -2 5 5
8 8 +2 8 A 10
9 9 +2 B 11
A 10 -2 8 8
B 11 -2 9 9
C 12 +2 12 E 14
D 13 +2 F 15
E 14 -2 C 12
F 15 -2 D 13

So instead of breaking all the binary out, my CCNA brain dump sheet could look like this:

0 +2 2
1 +2 3
2 -2 0
3 -2 1
4 6
8 A(10)
C(12) E(14)

Or even:

0 +2
1 +2
2 -2
3 -2

I’m just looking for a way to shortcut this so I don’t have to burn time in the test fiddling bits. Does this look like a reliable shortcut in your opinion?

Hi Joel,

First of all, I wouldn’t worry too much about this too much. The bit flipping is just one minor sub-topic of all IPv6 related stuff you can expect in the exam. It’s possible that you don’t get any questions about it so don’t stress about it too much. You can expect a lot of regular subnetting questions so that’s why I highly recommend a “cheat sheet” for that.

It seems this list would work. It might be a quick method if you’d get 10 bit flipping questions but if you only would get 1 question, it might be just as fast to quickly calculate it?

hex > binary > flip 7th bit> hex

So for example:

hex: 12 =binary: 0001 0010

flip 7th bit:

binary: 0001 0000

back to hex:


And we are done.

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Hi Rene,
In the case of multiple IPv6 addresses on a single interface, is it still possible to use the EUI-64 and how?

Hello Nadav,

That is no problem, the prefix is different so you will have a unique 128-bit IPv6 address. Here’s an example:

R1(config)#interface GigabitEthernet 0/1
R1(config-if)#ipv6 address 2001:DB8:1:1::/64 eui-64 
R1(config-if)#ipv6 address 2001:DB8:2:2::/64 eui-64 
R1(config-if)#ipv6 address 2001:DB8:3:3::/64 eui-64

Gets you:

R1#show ipv6 interface brief | include 2001


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Hi Rene,
Just wanted to know how will the host compute it’s interface ID when the IPv6 router sending the RA’s has a subnet greater that /64 (say /80). How will the host compute it’s interface ID with EUI-64 enabled.
I check on some vendor equipment and found that they don’t support subnet greater than /64.
I just want to know how will the host behave in this case.


Hello Nadav

This is an excellent question. According to RFC 5375 in section B.2.4 EUI-64 ‘u’ and ‘g’ Bits, it states the following:

When using subnet prefix lengths other than /64, the interface
identifier cannot be in Modified EUI-64 format as required by
[RFC4291]. However, nodes not aware that a prefix length other than
/64 is used might still think it’s an EUI-64; therefore, it’s prudent
to take into account the following points when setting the bits.

The document continues to state several considerations that should be checked carefully. In general, depending primarily on the vendor, using a prefix other than /64 may be possible in conjunction with EUI-64 IPv6 address computation, however, because it may have unpredictable results, it’s a good idea not to implement it.

I hope this has been helpful!



Im a little bit confused. The lesson first says that “The 7th bit represents the universal unique bit. A “built in” MAC address will always have this bit set to 0”. But then we proceed looking at examples that all have MAC with this bit at ‘1’. SO it can be either? Then why do we need to flip it at all? Seems depending on MAC we would be flipping it either to 0 or to 1. So what is it accomplishing? Seems at worst we stop having it been unique - original mac should be unique, if we change on bit it creates the (however remote) possibility that there is actually MAC deferring from out MAC by exactly one bit. In the end the question is what is this ‘universal unique bit’ anyway? Clearly no bit can be unique by definition (there are only 2 of them) nor could be their position of ‘7’. So whats in its name and what exactly is the need for it?

Hello Vadim

MAC addresses can either be universally administered addresses (UAA) or locally administered addresses (LAA). A UAA is one that has been uniquely assigned to a device by its manufacturer using the appropriate OUI and a unique remainder of the address. This ensures uniqueness worldwide.

An LAA is one that is manually assigned by an administrator or a protocol. An example of such a MAC address is the virtual MAC used in HSRP.

So how do you know if a MAC is universally or locally administered? By looking at the 7th bit. This bit is actually called the Universal/Local bit or U/L bit. When it is set to “1” the address is an LAA. When it is set to “0” it is a UAA.

Now, when applied to IPv6 and the EUI-64 process, the meaning of the U/L bit is reversed. Specifically, 1 means universal and 0 means local. This is why, when determining the IPv6 address from the MAC address, this bit is flipped. The result is, if you see this bit as a 1 in the IPv6 address, you know that the original MAC address that it was derived from was a UAA. If you see this bit as 0 in the IPv6 address, you know that the original MAC address that it was derived from was an LAA.

All of this is just a convention, a methodology that is simply applied by definition.

Yes, it can be either. But what it is originally tells us if the MAC address was universally or locally administered.

I hope this has been helpful!


Ok, thanks, now it comes in a new light. Or rather just ‘in light’ :slight_smile: . We simply have to flip the bit because IPV6 ‘thinks’ the 1 better than 0 identifies the address as unique. Thank you.

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Hi Guys - I’ve read that EUI-64 “automatically assign itself a unique 64-bit IPv6 interface identifier without the need for manual configuration or DHCP.”

I appreciate that the unique address is automatically generated but surely manual configuration still has to take place for each interface on your device? Eg.

Router(config-if)#ipv6 address 2001:1234:5678:abcd::/64 eui-64

If I have 32 interfaces on my router I still have to enter this command 32 times no? DHCP would truly automatically assign 32 addresses in an instant?



Hi Gareth

EUI-64 would more correctly be described as a mechanism that is used to generate unique IPv6 addresses. Now this mechanism can be used for global unicast addresses, or link local addresses. Whether or not you require user/administrator intervention or not depends on which of these is being implemented, as well as on the vendor of each particular network device/host.

For the case where you want to configure global unicast IPv6 addresses on the interfaces of a Cisco router, then yes, you will have to put in the command that you mentioned. However, there are situations where no user/administrator intervention is necessary. Some of these include:

  1. the configuration of a link local address on an IPv6 enabled interface of a Cisco Router
  2. the configuration of a global unicast IPv6 address on a TV connecting to a network since it has been preconfigured to autoconfig this address

Whether or not user intervention is required depends primarily on how the vendor has set up the config process. In any case, the EUI-64 is the mechanism that is used. If it has been enabled by default or not depends on the vendor of the device, and on the type of address being assigned.

I hope this has been helpful!


Thanks Laz - as I learn about global unicast and link local I guess this will throw up more questions!

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Hi, I usually come across this question;

"An engineer has configured a router to use EUI-64, and was asked to document the IPv6 address of the router. The router has the following interface parameters:

mac address C601.420F.0007
subnet 2001:DB8:0:1::/64

Which IPv6 addresses should the engineer add to the documentation?

Answer: 2001:DB8:0:1:C601:42FF:FE0F:7

It’s an exam question. Why isn’t the 7th bit reversed here? (C6)

Hello Alexis

You are correct, the answer should indeed be 2001:DB8:0:1:C401:42FF:FE0F:7 since the 7th bit of the MAC must be flipped.

I took a look at the related questions in our practice tests as well and made a correction that needed to be made.

I hope this has been helpful!


Thank you very much!

It’s been made clear to me.

There’s no special case with the U/L, right? I mean, you always have to reverse the bit?

Hello Alexis

Yes, you always have to reverse the bit. Remember that this bit simply signifies if the MAC address is hardwired (learned directly from the physical interface) or administratively assigned (such as a virtual MAC or a MAC directly configured by an administrator). The bit is set to 0 if it is hardwired, or 1 if it is configured. The reason we change the bit is explained in RFC 2373 where it says:

The motivation for inverting the “u” bit when forming the interface
identifier is to make it easy for system administrators to hand
configure local scope identifiers when hardware tokens are not
available. This is expected to be case for serial links, tunnel end-
points, etc. The alternative would have been for these to be of the
form 0200:0:0:1, 0200:0:0:2, etc., instead of the much simpler ::1,
::2, etc.

So the reason we flip the bit is more of a matter of convenience rather than a technical issue.

I hope this has been helpful!