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During preparation for an exam I came over two questions that didn't make sense to me.

"You are planning to subnet IPv4 addresses to use on a global network. The design must support creating two separate networks that allows for support up to 1000 hosts and maximize the number of networks that are avaible.

You need to identify the subnets that meet these requirements."

Network 1:

 1. 172.16.0.0/5
 2. 172.16.0.0/6
 3. 172.16.0.0/8

Network 2:

 1. 10.0.0.0/14
 2. 10.0.0.0/16
 3. 10.0.0.0/20

The correct answers are 2) for Network 1 and 1) for Network 2, but the calculations are not presented as a part of the solutions. I've been trying for a few days to work it out, but something in my brain seems to have crashed.

I'd be grateful if anyone could show me how to work from network ID, netmask and number hosts to determine which netmask is the best and provides the most subnets.

Sowndarya
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Truls TD
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  • You'll find lots of info right here: https://serverfault.com/questions/49765/how-does-ipv4-subnetting-work. I am also not sure if your question should be migrated to superuser since your question isn't about programming. – Noel Widmer Jun 08 '17 at 08:02
  • @NoelWidmer Thanks, I've went over that guide and found it very helpful in the past, but as pointed out I feel like I've hit a roadblock and I just can't get my head around how to calculate this. Also quite new to this site, so it should probably be moved - thanks! – Truls TD Jun 08 '17 at 08:05

1 Answers1

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10101100 00010000 00000000 000000 is the binary representation of the IP address 172.16.0.0. the subnet must be done in such a way so that atleast 1000 hosts can be allocated. We cannot touch the 172 and 16 address as it belongs to a different class. now looking at

00 00 00 00 and 00 00 00 00

if we subnet the 1st 6 places it will leading to

11 11 11 00 00 00 00 00 to 11 11 11 11 11 11 11 11

which is 172.16.252.0 to 172.16.255.255 giving us 256*4 = approx 1000 hosts hence we subnet using 172.16.0.0/6 but not others as it may lead to shortage or excess present available hosts Hope its helpful for the other but consider class as the second question belong to different class

kautilya hari
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  • Yes as it belong to class A ONLY X. 255.255.255 will be changed where X wont be – kautilya hari Jun 08 '17 at 12:25
  • So to use your example for the 10.0.0.0 network. It's expressed as `00001010 00000000 00000000 00000000` If we reserve the 1st 14 places, it will look like this `11 11 11 11 11 11 11 00 00 00 00 00` to `11 11 11 11 11 11 11 11 11 11 11 11` This leaves us with 10.255.252.0 to 10.255.255.255 which gives us 256*4 = approx 1000 hosts again. Is that right? – Truls TD Jun 08 '17 at 12:28
  • There u go... Prefect – kautilya hari Jun 08 '17 at 12:39
  • Network classes are dead, killed in 1993 by RFCs 1518 and 1519, which defined CIDR (_Classless_ Inter-Domain Routing). Modern networking doesn't use network classes, Please let them rest in peace. – Ron Maupin Jun 08 '17 at 15:00