[SpamCop-List] Re: SBCGlobal) Software Factory solutionsSFSFL and PACBell and Whoa007 @pacbell

[Blammo]

On 11 May 2005 N. Miller entered spamcop and left 
news:2a5l23mdxndq.dlg at news.spamcop.net:

> I like to use this for CIDR calculations:
> 

I finally figured out how to do that with the scientific calculator,
here I use the carot ^ for the button [x^y].

e.g. /24

2 ^ (32 - 24) = 256 - 1 = 255 

This gives you the number of addresses minus the one you already have. 
Converting to dotted quad is a little clumsy, you need to divide by 256 for 
any number greater than 255 to get each quad, (then subtract 1) adding the 
result to the first address.

Converting back probably makes this more clear:
e.g. 69.67.64.0 - 69.67.79.255
each dotted quad = 256, and we have to add one to include the "0" address. 
(79 - 64 + 1) * 256 = 4096
32 - (4096 log / 2 log) = 20
69.67.64.0 - 69.67.79.255 = 69.67.64.0/20

Another example: 69.67.0.0 - 69.68.255.255
I know this = 69.67.0.0/15

69.68.255.255 - 69.67.0.0 = (68 - 67 + 1) * 256 * 256 = 131072
Or I think you could expand that to say
(69 - 69 + 1) * (68 - 67 + 1) * (255 - 0 + 1) * (255 - 0 + 1) = 131072
32 - (131072 log / 2 log) = 15

converting back...

2 ^ (32 - 15) = 131072
131072 / 256 = 512 [0.0.0.255]
512 / 256 = 2      [0.0.255.0]
2 - 1 = 1          [0.1.0.0]
69.67.0.0 + 0.1.255.255 = 69.68.255.255

or hex(131072 - 1) = 1FFFF

CIDR seems complicated, but its simply the number of fixed bits in the 
address range. IPv4 contain 32 bits, so a range of one address would have 
32 fixed bits or /32.
2 ^ (32 - 32) = 1
binary is multiples of 2 (2^y), so
/32 = 1
/31 = 2
/30 = 4
/29 = 8
/28 = 16
-- 
| Ric
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