Re: Is 1111 1110 10 equal to 0xfe80 or

Alexandre Petrescu <alexandre.petrescu@gmail.com> Sat, 15 June 2019 15:03 UTC

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Subject: Re: Is 1111 1110 10 equal to 0xfe80 or
To: "Bless, Roland (TM)" <roland.bless@kit.edu>, Fred Baker <fredbaker.ietf@gmail.com>
Cc: IPv6 <ipv6@ietf.org>
References: <a71b00b7-0e0c-242a-b3f7-147f4c6b2eb0@gmail.com> <63DC2EEC-C456-4090-9242-6675F47B6351@gmail.com> <b71b9dfb-cd1e-b5cf-f0bf-885d479f6d29@gmail.com> <a30173da-3800-7ed6-88af-3400b6dc9c39@kit.edu>
From: Alexandre Petrescu <alexandre.petrescu@gmail.com>
Message-ID: <16364555-2d47-0ec5-c658-04193e19e154@gmail.com>
Date: Sat, 15 Jun 2019 17:02:55 +0200
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Le 07/06/2019 à 08:58, Bless, Roland (TM) a écrit :
> Hi Alex,
> 
> Am 07.06.19 um 07:50 schrieb Alexandre Petrescu:
>> Le 06/06/2019 à 23:41, Fred Baker a écrit :
>>>
>>>
>>>> On Jun 6, 2019, at 10:18 AM, Alexandre Petrescu
>>>> <alexandre.petrescu@gmail.com> wrote:
>>>>
>>>> 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.
>>>
>>> You're correct if those are the only bits. However, in 1111 1110 10
>>> you have to also include 00 0000, because it's a /10 prefix.
>>
>> ?
>>
>> 1111 1110 10 are 10 bits and they are the only bits.
> 
> Gee, it's only binary arithmetic. We use a nibble-wise textual
> representation with hex digits, so you have to fill it up to
> a multiple of 4 bits, i.e., adding two bits to the right gives you
> 12 bits, which nicely converts to 0xfe8. If you want 0xfe80 you
> need to add 00 0000 as Fred pointed out.
> 
>> I would indeed add the 00 0000 if they were 16bits, and then I would
>> write fe80::/16.
>>
>> But, since a few people say differently, I think I will need to look at
>> it deeper.  MAybe it is just a matter of language expression.
> 
> Sorry, but fe80:: doesn't imply fe80::/16, that's exactly the reason
> why we have the prefix notation. fe80::/10 means to expand to 128 bit
> and then use the leftmost 10 bits only. That's unambiguous.

Sorry, it is ambiguos.

Even if you expand it to 128, you still need only the 10bits.  The 
expansion to 128 bits has no particular meaning.  Were the 128bit 
expansion meaning anything; then we would not need to convert.

Rather, probably a new base is needed for IPv6 (a base is like binary, 
hexa, octal, etc).

Alex

> 
> Regards
>   Roland
>