Re: Universal/Local Bit

Hi Anantha,

That is a nice question IMO, Below is an excerpt from the RFC 4291 for IPv6
addressing architecture

"*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.*
* *
* IPv6 nodes are not required to validate that interface identifiers*
* created with modified EUI-64 tokens with the "u" bit set to universal*
* are unique.*
* *
* The use of the universal/local bit in the Modified EUI-64 format*
* identifier is to allow development of future technology that can take*
* advantage of interface identifiers with universal scope.* "

As I have understood it, there is not much significance of the U/L bit as of
now and it's just a way of providing network administrators a method of
manually deriving endpoint IP addresses .

HTH,
Ravi

On Tue, Jun 23, 2009 at 1:48 PM, Scott Morris<swm_at_emanon.com> wrote:
> heheheheh.... There is no need. :)
>
> It's an academic one! If you modify your MAC you're supposed to change
> that. IMHO, if you can't figure out there was something modified in
> order to go from 48 bits to 64 bits, you need therapy. But it's part
> of the spec. Go figure.
>
>
>
>
> *Scott Morris*, CCIE/x4/ (R&S/ISP-Dial/Security/Service Provider) #4713,
>
> JNCIE-M #153, JNCIS-ER, CISSP, et al.
>
> CCSI #21903, JNCI-M, JNCI-ER
>
> swm_at_emanon.com
>
>
> Knowledge is power.
>
> Power corrupts.
>
> Study hard and be Eeeeviiiil......
>
>
>
> Anantha Subramanian Natarajan wrote:
>> Hi All,
>>
>> I am trying to understand the need for inverting the U/L bit(7th MSB
bit)
>> to get the modfied EUI-64 address for IPV6.My understanding is,FFFE is
>> inserted in middle to increase the size from 48 bit mac address to 64 so
>> that we could be able to accomdate ipv6 /64 host spacing and couldn't
>> understand why we invert U/L bit.Thanks for the assistance.
>>
>> Regards
>> Anantha Subramanian Natarajan
>>
>>
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Received on Tue Jun 23 2009 - 15:02:57 ART