subnetting C class into /26 /25 /26, why can this be done?

[Daniel C. Sobral]

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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