Re: IPv6 Link Local Addresses

On Tue, 11 Jun 2019 at 10:42, Kerry Lynn <kerlyn@ieee.org> wrote:

> On Tue, Jun 11, 2019 at 9:05 AM Alexandre Petrescu <
> alexandre.petrescu@gmail.com> wrote:
>
>>
>>
>> Le 10/06/2019 à 18:44, Bob Hinden a écrit :
>> > Hi,
>> >
>> > RFC4291 defines Link Local Address as:
>> >
>> >   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         |
>> >     +----------+-------------------------+----------------------------+
>> >
>> >     Link-Local addresses are designed to be used for addressing on a
>> >     single link for purposes such as automatic address configuration,
>> >     neighbor discovery, or when no routers are present.
>> >
>> >     Routers must not forward any packets with Link-Local source or
>> >     destination addresses to other links.
>> >
>> > This means that link local addresses have a 10 bit prefix (1111111010)
>> followed by 54 bits of zeros.  That is it, nothing more.   Address with
>> different prefixes or with a 1111111010 prefix followed by non-zero 54 bits
>> are not link local addresses.
>> >
>> > This is not ambiguous.
>>
>> Bob,
>>
>> The RFC4291 link local addresses have a 10 bit prefix and 54bits of
>> zero.  That is fe80::/64.
>>
>> The IANA allocation of IPv6 link-local addresses is fe80::/10.
>>
>>  > fe80::/10    Link-Scoped Unicast     [RFC3513][RFC4291]
>>
>> In that sense, the IANA definition and the RFC4291 differ with respect
>> to the definition of link-local addresses.
>>
>> fe80::/10 and fe80::/64 are not the same thing.
>>
>> Alex
>>
>>
> Bob et al.
>
> Would a correct router implementation base its "must not forward" decision
> on the most-significant 10 bits or most significant 64 bits?
>

IMHO, /10.

The well-known prefix on a link is the /64.

The extra empty space between the two does have its uses (
https://www.freebsd.org/doc/en/books/developers-handbook/ipv6.html#ipv6-scope-index),
even if not officially sanctioned by a document.

Kerry
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