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TOPIC: Subnetting question [2048 or 8]???

Subnetting question [2048 or 8]??? 8 years 6 months ago #26198

  • kogula14
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Hi,

I met one confusion answer. My question as below:-

Practice Example #8B: 255.255.255.224 (/27)
172.16.0.0 = Network Address
255.255.255.224 = Subnet mask

Is it subnet = 2^11 = 2048 OR 2^3 = 8 ?????

* According to CCNA book Todd Lammle, Sixth Edition answer is 2048.
Which one correct ???

Currently im taking CCNA course and we are referring to this book to study in class. My letcurer also confused of this problem. He said MAYBE its wrong. Its really confusing us.

Please anyone help me on this. Your kind help will I appreciate.

Thank u.
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Re: Subnetting question [2048 or 8]??? 8 years 6 months ago #26202

  • SteveP
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The standard mask for this class B address is 255.255.0.0 so you have borrowed 11 bits to create the 255.255.255.224 mask. In this circumstance, I'd say that you have 2^11 = 2048 possible subnets (minus the first and last, depending on your configuration) and 2^5 = 32 possible hosts (again, minus the first and last) on each subnet.
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Re: Subnetting question [2048 or 8]??? 8 years 6 months ago #26206

  • kogula14
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but why when we do a subnetting calculation, we use 2^3=8 subnetworks and its the answer..

why?
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Re: Subnetting question [2048 or 8]??? 8 years 6 months ago #26210

  • Elohim
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Because a very long time ago, you could not use the all 0 and all 1 subnetworks, so the the calculation back then would be (2^n) - 2, but with the advent of ip subnet zero and it is on by default in the cisco routers, you can now drop the (-2) from the formula, thus if you have 3 bits for the subnetworks, you would have 8 subnetworks.
but why when we do a subnetting calculation, we use 2^3=8 subnetworks and its the answer..

why?
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Re: Subnetting question [2048 or 8]??? 8 years 6 months ago #26215

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but why when we do a subnetting calculation, we use 2^3=8 subnetworks and its the answer..
The whole point about subnetting is the number of bits borrowed (and remaining). The number of subnetworks is calculated by 2^(borrowed bits) and the number of hosts by 2^(remaining bits). As Elohim explained, and I indicated, the number of usable subnetworks may need to be reduced by 2 depending upon the configuration. I agree that ip subnet zero is the default now. The number of usable hosts is 2 less than the calculated total because the first is the network address and the final is the broadcast address. Neither can be used as a host address.
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Re: Subnetting question [2048 or 8]??? 8 years 6 months ago #26226

  • vanz04
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hi hope anyone can help me. im a little bit confuse of how to subnet(vlsm) this type scenario. hope anyone can show me a binary form for this. your answers are much appreciated.

given an ip address of 172.16.0.0/16

i know that the first network would be 172.16.0.0/23 convert to binary and that would be x.x.00000001.11111111. how do i subnet /23 to the 2nd network (/24). hope you can show the binary form.

3 networks
500 host - 1st network
200 host - 2nd network
100 host - 3rd network
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