IP subnetting
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Posted by RIckH on July 16, 2007, 8:40 am
Please log in for more thread options 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. Thanks for the help | |||||||||||||||||||||||||
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Posted by pk on July 16, 2007, 9:17 am
Please log in for more thread options 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.
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Posted by Gabriele Beltrame on July 16, 2007, 9:22 am
Please log in for more thread options
> Can someone please explain how the answer to this question is
Hi,
> 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. > > Thanks for the help > 2^(28-20) = 256 subnets (with ip subnet-zero) 172.17.32.0/20 172.17.32.0-172.17.47.255 172.17.32.0 (16 subnets) 172.17.32.0-172.17.32.15 172.17.32.16-172.17.32.31 .... .... 172.17.32.240-172.17.32.255 172.17.33.0 (16 subnets) 172.17.33.0-172.17.33.15 172.17.33.16-172.17.33.31 .... .... 172.17.33.240-172.17.33.255 ..... ..... 172.17.47.0 (16 subnets) 172.17.47.0-172.17.47.15 172.17.47.16-172.17.47.31 .... .... 172.17.47.240-172.17.47.255 Regards, Gabriele | |||||||||||||||||||||||||
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Posted by pk on July 16, 2007, 9:23 am
Please log in for more thread options 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. | |||||||||||||||||||||||||
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Posted by RIckH on July 16, 2007, 3:16 pm
Please log in for more thread options > 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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> 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.