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**posted on**

- Ira Gladnick

June 2, 2008, 3:43 am

Would anyone happen to have a pointer to a proof of how the "magic

number" algorithm for finding a subnet number's "interesting" octet

decimal value works? I.e., WHY does it work, mathematically speaking?

Algorithm is: subtract the mask's "interesting" (non-255) decimal

number from 256, which yields the "magic number". Then find the

largest multiple of the "magic number" that is less than the

"interesting" decimal value in question--this is the "interesting"

decimal number for the subnet address.

number" algorithm for finding a subnet number's "interesting" octet

decimal value works? I.e., WHY does it work, mathematically speaking?

Algorithm is: subtract the mask's "interesting" (non-255) decimal

number from 256, which yields the "magic number". Then find the

largest multiple of the "magic number" that is less than the

"interesting" decimal value in question--this is the "interesting"

decimal number for the subnet address.

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