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Re: [OT] CIDR Mask calculation



On Thu, Oct 12, 2006 at 10:38:05AM -0600, Ashley M. Kirchner wrote:
> ... I'm trying to figure out the CIDR mask or bits for a particular
> network.  Specifically: 219.128.0.0 - 219.137.255.255

The easiest way to do this is to convert to binary and look for the
first "variant bit"--the first bit that changes in the two address
ranges.  This marks the change from the network and device fields
of the subnet.

If you don't do decimal<>binary conversions in your head, a simple way
to do this to use 'dc', setting the output to base 2.  For instance:

  $ dc
  2
  o
  128
  p
  10000000
  255
  p
  11111111

Anyway, the addresses you gave, converted to binary, are:

  219.128.0.0    : 11011011.10000000.00000000.00000000
  219.137.255.255: 11011011.10001001.11111111.11111111
                               ^
The last bit that is invariant between the two is marked with the caret;
converting back to a decimal dotted-quad (you can use 'dc' again--to set
the input to binary, simply change the 'o' operand in the above example
to an 'i', then input the binary value and print.)

  Mask
  255.240.0.0    : 11111111.11110000.00000000.00000000

or, in conventional notation, 219.128.0.0/12.

Cheers,
--
	Dave Ihnat
	President, DMINET Consulting, Inc.
	dihnat dminet com


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