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EUI-64

zuppetta
Level 1
Level 1

Hi everybody,

someone knows why in the process of building IPV6 unicast address through EUI-64 the seventh bit from left must be complemented ?? I can't understand .....

thanks

4 Replies 4

Marcin Latosiewicz
Cisco Employee
Cisco Employee

Paolo,

You are refrring to 7th bit of mac address the U/L bit not the 7th bit of IPv6 address I guess :-)

You need to look at bit side of things. It makes thing easier in notation.

It's described here:

http://en.wikipedia.org/wiki/IPv6_address#Modified_EUI-64

Take a very simple MAC address and convert it to EUI-64 with and without the bit flipped.

You will see that it's "easier" to write it down when it's flipped.

M.

lgijssel
Level 9
Level 9

You may also want to check on the theory behind the U/L bit in mac addresses. Wikipedia is an excellent source.

Regards,

Leo

SunilKhanna
Level 1
Level 1

Hi Paolo,

hope this helps you

Understanding IPv6 EUI-64 Bit Address

Regards,

Sunil Khanna

PS: Please rate the helpful post.

Regards, Sunil Khanna

Phillip Remaker
Cisco Employee
Cisco Employee

This is explained in RFC4291 Section 2.5.1.  In short, it is so that the implicit modified EUI-64 portion of manually assigned addresses (such as 2001:db8::1) will be recognized as locally administered.

http://tools.ietf.org/html/rfc4291#section-2.5.1

Relevant text:

   The motivation for inverting the "u" bit when forming an interface
   identifier is to make it easy for system administrators to hand
   configure non-global identifiers when hardware tokens are not
   available.  This is expected to be the case for serial links and
   tunnel end-points, for example.  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 0:0:0:1, 0:0:0:2, etc.
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