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TOPIC: do we need to minus 2 from subnet bit ???

do we need to minus 2 from subnet bit ??? 9 years 5 months ago #28420

  • htetnaing
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Let's say, I've 3 bits of subnet bit so my total subnet would be 2 to the power of n which is 3 so 8 subnet networks. Is that correct ?? I've read in an article that in order to calculate total subnet networks we have to minus 2 from 2 to the power. I'm confused. If i minus 2 from my previous subnet then I'd get 6 subnet networks instead of 8. Hope someone would kindly answer my question and thanks all in advance. Oh , plus i'm a newbie to this website. so umm hi everyone. Been reading lots of article on this site and give 100/100 . it's awesome ! keep it up guys and thanks for providing such valuable articles.
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Re: do we need to minus 2 from subnet bit ??? 9 years 5 months ago #28426

  • TheBishop
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You're getting the number of subnet networks and the number of available hosts within a subnet mixed up. As you said, the number of subnets is 2 to the power n, where n is the number of bits you have borrowed from the host portion of the IP address. You've got the understanding of that part right. The second type of subnet problem is to say how many hosts you can have in a given subnet. That is where you work out how many bits you have for the host addresses, use your 2 to the power n again to work out the total number of possible hosts that gives you, but then you subtract 2. The reason you subtract 2 is that the subnet is effectively a little IP network of its own, and in every IP network the highest address is reserved for broadcasts and by definition (in Cisco exam questions) you don't use the lowes possible address either (because that is used to represent the network). So because these two addresses are reserved and cannot be used for real hosts you subtract two. I hope that helps
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