Subject: Seeking clarifying information on RFC 4291 and link-local address format ...

From: "John Spence" <[email-masked]>
Date: Tue, 11 Jul 2006 12:11:45 -0400

Reading RFC 4291, I am not 100% clear on the format for link-local
addresses.  In some places the document specifies "fe80::/10", which I
read as "the first ten bits must be 1111 1110 10, the remaining 54 bits
can be anything, up to the /64 bit boundary .  Later in the document I
see this:

2.5.6.  Link-Local IPv6 Unicast Addresses
 
   Link-Local addresses are for use on a single link.  Link-Local
   addresses have the following format:
 
   |   10     |
   |  bits    |         54 bits         |          64 bits           |
   +----------+-------------------------+----------------------------+
   |1111111010|           0             |       interface ID         |

Which I read as "the first 64 bits are always fe80::", but then I would
think we'd write that as "fe80::/64".

So, do those middle 54 bits have to be zero, or can they be anything?

Thanks for any insight.

John Spence, Command Information

[email-masked] <mailto:[email-masked]> 

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From: Pars MUTAF <[email-masked]>
Date: Tue, 11 Jul 2006 21:26:39 +0200

Hello,

>    |  bits    |         54 bits         |          64 bits           |

I ping6ed my Linux link-local address with those 54 bits equaling 0:

pars@lazylizard:~$ ping6 -I eth1 fe80:0:0::2c0:9fff:feef:ffb4
PING fe80:0:0::2c0:9fff:feef:ffb4(fe80::2c0:9fff:feef:ffb4) from
fe80::2c0:9fff:feef:ffb4 eth1: 56 data bytes
64 bytes from fe80::2c0:9fff:feef:ffb4: icmp_seq=1 ttl=64 time=0.041 ms
64 bytes from fe80::2c0:9fff:feef:ffb4: icmp_seq=2 ttl=64 time=0.034 ms

Then, I tried to ping6 my link-local address by modifying some of
your 54 bits:

pars@lazylizard:~$ ping6 -I eth1 fe80:0:1::2c0:9fff:feef:ffb4
connect: Network is unreachable
pars@lazylizard:~$

Looks like Linux likes them when they are zero.
:-/
I'm not sure what this means. I hope this helps.

Regards,
pars

> [email-masked] <mailto:[email-masked]>

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From: Bob Hinden <[email-masked]>
Date: Tue, 11 Jul 2006 18:24:55 -0400

John,

> Which I read as “the first 64 bits are always fe80::”, but then I  
> would think we’d write that as “fe80::/64”.
>
They must be zero.

The prefix that identifies IPv6 link local addresses is FE80::/10 as  
defined in Section 2.4.  All of the addresses under this prefix have  
link-scope.

The format of link local addresses is defined in Section 2.5.6 and it  
requires that there be 54 bits of zeros between the prefix and the  
Interface ID.

In the future we could define some other types of address that have  
link-scope and these would also be in the FE80::/10 prefix, but now  
there is only one type.

Bob

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From: Brian Haberman <[email-masked]>
Date: Tue, 11 Jul 2006 13:42:23 -0400

Hi John,
       The specification reserves FE80::/10 for any link-local address 
format.  The only currently used instantiation is the FE80::/64 prefix. 
  In other words, a new RFC could define alternative formats for the 
internal 54 bits underneath the FE80::/10 prefix.

Regards,
Brian

On Jul 11, 2006, at 12:11, John Spence wrote:

> addresses.  In some places the document specifies “fe80::/10”, which I 
> read as “the first ten bits must be 1111 1110 10, the remaining 54 
> bits can be anything, up to the /64 bit boundary .  Later in the 
> document I see this:
>  
> [email-masked]
>