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

Tom Herbert <tom@herbertland.com> Thu, 06 June 2019 17:48 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Thu, 06 Jun 2019 10:48:38 -0700
Message-ID: <CALx6S37zVNDEb5863KuUEETyxuKhyGU1Vc7__qZQ2Mkyb0tzyg@mail.gmail.com>
Subject: Re: Is 1111 1110 10 equal to 0xfe80 or 0x3fa?
To: Alexandre Petrescu <alexandre.petrescu@gmail.com>
Cc: IPv6 <ipv6@ietf.org>
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On Thu, Jun 6, 2019 at 10:18 AM Alexandre Petrescu
<alexandre.petrescu@gmail.com> wrote:
>
> Hi 6MANners,
>
> In private discussion we disagree about this point:
>
> I maintain that 1111 1110 10 equals 0x3fa, because that's what my Windows Calculator says: I type 1111 1110 10 and it converts to 0x3fa.
>
> On another hand, I am rhetorically asked how can 1111 1110 10 be 0x3fa?
>
Alexandre, presumably you're referring to the table in section 2.4. In
this context 1111 1110 10 is a binary prefix, not an ordinal number,
so it is neither equal to 0xf3a nor 0xfe80. It is equal to FE80::/10
in IPv6 notation as inidicated.

Tom

> (the 1111 1110 10 are the 10 leading bits of the IPv6 link local addresses, which is familiarly known to start with an fe80).
>
> On my side, this is a difficulty to understand this 0xfe80, especially since 1111 1110 10 is so printed in Figure in RFC4291.
>
> Alex
>
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