Date: Wed, 27 Aug 2003 16:41:28 -0300 From: "Daniel C. Sobral" <dcs@tcoip.com.br> To: Christoffer Pio <cp@cvt.dk> Cc: freebsd-net@freebsd.org Subject: Re: subnetting C class into /26 /25 /26, why can this be done? Message-ID: <3F4D0968.5050800@tcoip.com.br> In-Reply-To: <3F4CBA6A.5AA7DEB0@cvt.dk> References: <3F4CBA6A.5AA7DEB0@cvt.dk>
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Christoffer Pio wrote:
> Is it not possible to subnet a C class into 3 nets, like
>
> 0-63
> 64-191 <-- Offending network (?)
> 192-255
No.
> If so, why is this?
Because it there is no mask that will fit. In binary, you have:
0 0000 0000
63 0011 1111
64 0100 0000
127 0111 1111
128 1000 0000
191 1011 1111
192 1100 0000
255 1111 1111
Now, how do masks work? Where there are 1's in a mask, the address is
supposed to never change. Where there are 0's, any value is accepted.
The problem you have is that between 64 and 191 there is absolutely no
digits that won't change (in fact, you can see that the 191 is the very
opposite of 64 in every bit). So any network that includes both 64 and
191 will necessarily include everything from 0 to 255.
Mind you /25 and /26 are just a simplification of the way masks are
almost always used. Usually, the left side of the mask is all 1's up to
a certain point, and from there to the end there are just 0's. It is
_possible_ to use masks in a different way, but I have only seen such
configurations as a result of errors.
For example, it would be theoretically possible to have a network
composed of 0-63 and 128-191. Fortunately, I haven't seen anyone do that
intentionally. :-)
So /25 means that the first 25 bits of the 32 bits that compose an
address are 1's, and the remaining are 0's.
(and anyone picking on my usage of "first", please relate it to how
people write numbers in oriental languages and arabic -- no sense in
setting the bar too low :)
--
Daniel C. Sobral (8-DCS)
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E-mail: Daniel.Capo@tco.net.br
Daniel.Sobral@tcoip.com.br
dcs@tcoip.com.br
Outros:
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capo@notorious.bsdconspiracy.net
We Klingons believe as you do -- the sick should die. Only the strong
should live.
-- Kras, "Friday's Child", stardate 3497.2
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