Re: Is 1111 1110 10 equal to 0xfe80 or 0x3fa?

[Tom Herbert <[e-mail]>]

On Fri, Jun 7, 2019 at 12:30 PM Mudric, Dusan (Dusan) <[e-mail]> wrote:
>
> >
> > Message: 3
> > Date: Fri, 7 Jun 2019 10:53:28 -0700
> > From: Fred Baker <[e-mail]>
> > To: Alexandre Petrescu <[e-mail]>
> > Cc: IPv6 <[e-mail]>
> > Subject: Re: Is 1111 1110 10 equal to 0xfe80 or 0x3fa?
> > Message-ID: <[e-mail]>
> > Content-Type: text/plain; charset="us-ascii"
> >
> >
> >
> > If I have prefix fe80::/10, as described in RFC 4291, the next bit is bit 11. Doing
> > the same subdivision of the prefix is fe80::/11 and fea0::/11.
> [Dusan] The hexadecimal definition for LL address is not syntactically correct. The binary 10 bit prefix 1111111010 cannot be presented as hexadecimal FE80::/10. It is rather a range FE80::/10 - FEBF::/10. In this notation, FE80::/10 = FEBF::/10,  because the first 10 bits are equal and other 6 should be ignored. 111 1111010 can be defined as FE80::/10 only if every time it is also mentioned that the trailing 6 bits are all zero.

By that logic, we'd have to mention that the trailing 118 bits are
zero.  E.g. FE80::/10 == FEBD:F676:BBBB:C654:FEBD:F676:BBBB:C654/10
also. It's obviously convenient canonical notication to express all
the trailing bits as zeroes for a prefix, but not required. For
instance, ifconfig shows my host address as fe80::ac2f:ea58:94a:438/64
which in one string indicates both a fully qualified address and it's
prefix bits.

Tom

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