Subject: link/site local prefixes

From: Dirk Ooms <[email-masked]>
Date: Thu, 21 Sep 2000 11:49:23 +0200

Probably this is something that is asked many times before, but I am
completely new to the subject and there is no flat mailing list archive
to do a search.

As far as I understand the two below extracts of RFC2373 contain a
contradiction.  So is the prefix of link-local 1111111010/10 (fraction
is 1/1024) or 111111101000::/64 (fraction is 1/2**64)? Same question for
site-local.

dirk

    Allocation                            Prefix         Fraction of
                                          (binary)       Address Space
    -----------------------------------   --------       -------------
    Link-Local Unicast Addresses          1111 1110 10   1/1024

...

2.5.8 Local-Use IPv6 Unicast Addresses

   There are two types of local-use unicast addresses defined.  These
   are Link-Local and Site-Local.  The Link-Local is for use on a single
   link and the Site-Local is for use in a single site.  Link-Local
   addresses have the following format:

   |   10     |
   |  bits    |        54 bits          |          64 bits           |
   +----------+-------------------------+----------------------------+
   |1111111010|           0             |       interface ID         |
   +----------+-------------------------+----------------------------+
--------------------------------------------------------------------
IETF IPng Working Group Mailing List
IPng Home Page:                      http://playground.sun.com/ipng
FTP archive:                      ftp://playground.sun.com/pub/ipng
Direct all administrative requests to [email-masked]
--------------------------------------------------------------------

---

From: Francis Dupont <[email-masked]>
Date: Thu, 21 Sep 2000 12:34:06 +0200

In your previous mail you wrote:

   As far as I understand the two below extracts of RFC2373 contain a
   contradiction.  So is the prefix of link-local 1111111010/10 (fraction
   is 1/1024) or 111111101000::/64 (fraction is 1/2**64)?

=> it is fe80::/64 for all the links using a 64 bit interface ID
(ie. all the links where IPv6 transport is currently defined).
For site-locals the prefix is fec0::/48 because of the 16 bits for
routing inside the site.

[email-masked]

---

From: "Matt Crawford" <[email-masked]>
Date: Thu, 21 Sep 2000 08:33:44 -0500

It's not quite a contradiction.  The first excerpt says that every
address matching fe80::/10 is reserved for link-local use.  The
second says that actual link-local addresses will use only the tiny
fraction of that space which matches fe80::/64.  You could consider
the space in which the 54 bits just after the first 10 are not all
zero to be either "wasted" or "reserved for future link-local
variants".
--------------------------------------------------------------------
IETF IPng Working Group Mailing List
IPng Home Page:                      http://playground.sun.com/ipng
FTP archive:                      ftp://playground.sun.com/pub/ipng
Direct all administrative requests to [email-masked]
--------------------------------------------------------------------

---

From: Dirk Ooms <[email-masked]>
Date: Thu, 21 Sep 2000 16:36:43 +0200

Matt,

The second excerpt says Link-Local addresses have the format fe80::/64,
the first excerpt says 1/1024 of the address space is available for
Link-Local addresses. To me this is a contradiction. It wouldn't have
been a contradiction if the second excerpt would mention something like
'Link-Local of type x'.

Anyway, I understand what is meant.  Thanks,
dirk