Subject: About Link-Local address prefix.

From: "SEENYEN,GENE (HP-Australia,ex3)" <[email-masked]>
Date: Wed, 21 Aug 2002 13:20:30 +1000

Hi All,
	  I hope someone would be able to help me out here. I have a query
regarding what is documented and known as the prefix for the Link-Local
addresses. It is generally defined as FE80::/10 but looking at RFC2464 it
says in section 5.

5 Link-Local Addresses 
The IPv6 link-local address [AARCH] for an Ethernet interface is formed by
appending the Interface Identifier, as defined above, to the prefix
FE80::/64. 
       10 bits            54 bits                  64 bits
     +----------+-----------------------+----------------------------+
     |1111111010|         (zeros)       |    Interface Identifier    |
     +----------+-----------------------+----------------------------+

Here the prefix is FE80::/64.  I know that FE80::/10 and FE80::/64 are the
same and that in practice FE80::/10 currently means FE80:/64 with the node's
EUI-64 in the bottom 64 bits but what I am not sure is how the /64 is
achieved. Does the /64 refer to the 10 MSB bits + the 54 bits which are all
0 or does the /64 mean the 64 Interface identifier bits? Also why use either
FE80::/10 or FE80::/64 as the Link local address instead of having just one?

Thanks for the help

Gene

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From: Robert Elz <[email-masked]>
Date: Wed, 21 Aug 2002 12:57:55 +0700

Date:        Wed, 21 Aug 2002 13:20:30 +1000
    From:        "SEENYEN,GENE (HP-Australia,ex3)" <[email-masked]>
    Message-ID:  <[email-masked]>

  | Does the /64 refer to the 10 MSB bits + the 54 bits which are all 0

Yes, the prefix length always counts bits from the leftmost in a 128
bit address, regardless of how many of those have known fixed values
in any particular case.

  | Also why use either FE80::/10 or FE80::/64 as the Link local
  | address instead of having just one?

It depends upon the context which is better.

All addresses that are FE80::/10 are link local.   So, if you are wanting
to test whether an addr is LL or not, that's what you'd be using.

On the other hand, a prefix assigned to (most) nets is a /64, so when
assigning a LL prefix to a LAN (or other net) you need to assign the /64
(you can, if you like, think of this as being one subnet out of the LL
space - except that it is the only one defined, none of the others are
available for use, or would mean anything if they were).

kre