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Posted by RIckH on July 16, 2007, 3:16 pm
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> RIckH wrote:
> > Can someone please explain how the answer to this question is
> > obtained ?
>
> > How many subnets are gained by subnetting 172.17.32.0/20 into multiple/
> > 28 subnets?
>
> > They answer is 256, but I can not figure out how to come up with that
> > answer.
>
> If you keep 172.17.32.0/20 as is, you have just one big network (of course).
>
> If you use a /28 mask, then you have the following subnets (relative to the
> original address and mask):
>
> 172.17.32.0/28 .00100000.0000 0000 (last two octets, space at /28)
> 172.17.32.16/28 .00100000.0001 0000
> 172.17.32.32/28 .00100000.0010 0000
> 172.17.32.48/28 .00100000.0011 0000
> ...
> 172.17.32.240/28 .00100000.1111 0000 (16 subnets so far)
>
> then:
>
> 172.17.33.0/28 .00100001.0000 0000
> ...
> 172.17.33.240/28 .00100001.1111 0000 (other 16 subnets )
>
> and so on until
>
> 172.17.47.0/28 .00101111.0000 0000
> ...
> 172.17.47.240/28 .00101111.1111 0000
>
> So, you have 16*16 subnets (eg, 256) if you use a /28 mask.
>
> You can also see this very quickly if you think that you gain 8 bits for the
> subnet part (/20 -> /28), thus you have 2^8 = 256 subnets.
>
> Ok, strictly speaking, you "gain" 255 subnets (you had one, and now you have
> 256), but I hope you got the idea.
Thanks, That makes perfect sense now.
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