Calculate PREFIX from subnet mask in bootscripts

Kevin P. Fleming kpfleming at
Tue Oct 19 07:36:18 PDT 2004

Gerard Beekmans wrote:

> So never mind my question about what is. It's /30
> according to this table. Good enough for me. Though I'd still appreciate
> the math behind it if anybody can explain it.

The math is pretty easy:

An IP address is 32 bits long. Some portion of those bits, starting from 
the beginning, are used to separate the address into "network" and 
"host" addresses.

If you turn your netmask of into binary, it becomes:

1111-1111 1111-1111 1111-1111 1111-1100

If you then count the number of 1-bits starting from the beginning, you 
get 30, so this netmask is equivalent to a VLSM prefix of /30.

The common /24 prefix is:

1111-1111 1111-1111 1111-1111 0000-0000

And so on.

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