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How do I find the x th subnet of X.X.X.X /XX (300 th subnet of 172.16.0.0 /29)?

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up vote 3 down vote accepted
  1. calculate how many address is there in /29 subnet (full IP has 32 bits, so you're left with 3 bits for subnet), 2 to the power of 3 is 8. You have 8 address per subnet.

  2. see how many /29 subnets can you have in a one /24 subnet (172.16.0.0-172.16.0.255), it 256/8 so 32 /29 subnets

  3. Divide 300 by 32 and get 9.75 so the 300th /29 subnet will be in the 10th /24 subnet (172.16.9.0-172.16.9.255), beacuse you have to take 9 full /24 subnets and part of 10th

  4. multiply 9*32 and you get that the 9th subnet ends with 288th /29 subnet, so you have to take 12 more /29 subnets from the 172.16.9.0/24, which is (12-1)*8 so the 300th is 172.16.9.88.0/29

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I forgot that 0-255 is composed of 256 ip addresses. That's why my count was off. Thanks. – HardwareMuch Sep 20 '12 at 22:44

You calculate it. A /29 subnet has 8 addresses (3 bit host address), so how many addresses do n-1 subnets have? And how many of those addresses fit in one octet?

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