Ipv6 auto address assignment in EUI-64 format

[anshubathla]

What is the sigificance of changing the 7 bit in MAC address while assigning this  as interface identifier in IPv6?

[Harold Ritter]

Hi,

Here's the excerpt from section 4 of RFC2464 explaining the reason for it.

 The Interface Identifier is then formed from the EUI-64 by
 complementing the "Universal/Local" (U/L) bit, which is the next-to-
 lowest order bit of the first octet of the EUI-64.  Complementing
 this bit will generally change a 0 value to a 1, since an interface's
 built-in address is expected to be from a universally administered
 address space and hence have a globally unique value.  A universally
 administered IEEE 802 address or an EUI-64 is signified by a 0 in the
 U/L bit position, while a globally unique IPv6 Interface Identifier
 is signified by a 1 in the corresponding position.

http://tools.ietf.org/html/rfc2464

Hope this helps

[Reza Sharifi]

https://supportforums.cisco.com/docs/DOC-24485

[anshubathla]

Thanks for sharing the link,

I already read that link but still not able to understand why it is nessary to invert the 7th bit because MAC is itself a unique address.

For MAC address in OUI field  if this bit is 0 then it is a global unique adresss assigned by IEEE,so  what is the need to invert this bit while assigning in IPv6

It will work without inverting also.

Please correct me if I ma wrong.

[Harold Ritter · Accepted solution]

Even better explanation in section 2.5.1 of RFC2373:

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.

http://tools.ietf.org/html/rfc2373#section-2.5.1

Regards