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

[Yucel Guven <[e-mail]>]

RFC 4291 sec.2.5.6  does NOT define a range. 10 bits are fixed, 54 bits are
zero, and the remaining 64 bits are for interface ID, that's all.

The topic of this thread/chain is: "Is 1111 1110 10 equal to 0xfe80 or
0x3fa?"
If you want to make comparison, you must take care of all 128 bits of the
0xfe80,
but not only a part of it.
In fact it should be written as '0xfe800000000000000000000000000000'
because it's a one complete number.

1111: this is 'f'
1110: this is 'e'
10??: Why do you take only two bits? Your answer will be: "Because it's
/10".

That approach is not true when you convert the number back to hexadecimal
notation.
You MUST think/care of ALL the 128bits, not only a part of it,
otherwise you destroy/change the original number.

Reg.'s
Yucel

On Mon, Jun 10, 2019 at 6:09 PM Mudric, Dusan (Dusan) <[e-mail]>
wrote:

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> *   From: Yucel Guven <[e-mail] <[e-mail]>>
> Sent: Monday, June 10, 2019 12:47 PM To: Mudric, Dusan (Dusan)
> <[e-mail] <[e-mail]>> Cc: Tom Herbert
> <[e-mail] <[e-mail]>>; [e-mail] <[e-mail]>
> Subject: Re: Is 1111 1110 10 equal to 0xfe80 or 0x3fa?   No need to define
> as a range. When you specify the prefix-length, it already defines a range.
> e.g. FE80::/10 (absolutely not  FE8::/10)  has the range of
> fe80:0000:0000:0000::/10 - febf:ffff:ffff:ffff::/10 [Dusan] I agree that
> FE80::/10 is the FE80::/10 – FEBF::/10 range. However, it is not define
> like a range in RFC 4291 and if often misinterpreted as just one value of
> FE80:0000:0000:0000. The question is how to write it to make it clear this
> definition is a range?   https://tools.ietf.org/html/rfc4291#section-2.5.6
> <https://tools.ietf.org/html/rfc4291#section-2.5.6> defines LL as the
> address with 64 bit FE80:0000:0000:0000 prefix, not as the
> fe80:0000:0000:0000::/10 - febf:ffff:ffff:ffff::/10 range. There are
> applications that need more flexibility for the LL prefix, like
> draft-petrescu-6man-ll-prefix-len. For these applications, FE80::/10 would
> be defined as LL identifier, not a prefix. The LL prefix would start with
> LL identifier and can have a variable length.   Is there any strong reason
> to keep 54 bits of zeros in this definition, other than backward
> compatibility?    |   10     |    |  bits    |         54 bits
> |          64 bits           |
> +----------+-------------------------+----------------------------+
> |1111111010|           0             |       interface ID         |
> +----------+-------------------------+----------------------------+
> Reg.'s Yucel   On Mon, Jun 10, 2019 at 3:58 PM Mudric, Dusan (Dusan)
> <[e-mail] <[e-mail]>> wrote: > On Friday, June 7, 2019 at
> 3:48 PM Tom Herbert > <[e-mail] <[e-mail]>> wrote > >
> > > > > > > Message: 3 > > > Date: Fri, 7 Jun 2019 10:53:28 -0700 > > >
> From: Fred Baker <[e-mail] <[e-mail]>> > >
> > To: Alexandre Petrescu <[e-mail]
> <[e-mail]>> > > > Cc: IPv6 <[e-mail]
> <[e-mail]>> > > > Subject: Re: Is 1111 1110 10 equal to 0xfe80 or
> 0x3fa? > > > Message-ID: <[e-mail]
> <[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.
> [Dusan] In this example fe80::ac2f:ea58:94a:438/64 is LL address with
> 0xfe80 0x0000 0x0000 0x0000 prefix. Based on LL prefix definition, this LL
> address can have a value of feab::ac2f:ea58:94a:438/64 and still have the
> binary 10 bit prefix 1111111010 May be LL address can be defined in hex
> notation as a range FE8::/10 - FEB::/10? This range always has the same
> well know LL identifier, the binary 10 bit prefix 1111111010. > > Tom >
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