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assitance for subnetting

Discussion in 'New Members Introduction' started by durairaj29, Sep 17, 2007.

  1. durairaj29

    durairaj29 New Member

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    Hi,
    iam persuing ccna, this is my 2nd post, i had gone through some of the websites, from there i got a question which has mentioned below, iam confused from the answer below, pls i need assistance from any one

    Question: You are designing a subnet mask for the 10.0.0.0 network. You want 3400 subnets with up to 3000 hosts on each subnet. What subnet mask should you use?

    Answer: 255.255.240.0
     
  2. GiddyG

    GiddyG Terabyte Poster Gold Member

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    Welcome to CF.

    This shows how much I have left to learn, coz I always assumed that a subnet could only have up to 255 hosts.

    Sorry I can't help. :rolleyes:
     
  3. ciscopaul

    ciscopaul Bit Poster

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    Hi,

    A traditional Class A subnet would have a mask of 255.0.0.0 so no bits are stolen for subnetting. If you want to subnet it down you have to steal host bits. We count subnet and host bits stolen using powers of two thus:

    2
    4
    8
    16
    32
    64
    128
    256
    512
    1024
    2048
    4096
    and so on.

    To get 3400 subnets you tick down until you reach the required number which in your case is 4096 because that will give you enough subnets. That is 12 places or 12 binary bits. 12 binary bits written out as subnet bits is 255.240 which you have to add to the current 255 network bits so your mask is now:

    255.255.240.0

    You have stolen 8 bits from the 2nd octet and 4 from the third to make your subnets. You now have another 12 bits left for hosts. You have to take 2 away from the host bits giving your 4096 hosts minus 2 = 4094 hosts per subnet.

    This is presuming subnet zero is allowed.
    http://www.subnetting-secrets.com
    Sorry this is a bit long. I have some lessons on www.subnetting-secrets.com which will help. You need to nail this because subnetting is fundamental to all networking jobs and certs.

    All the best
     
  4. GiddyG

    GiddyG Terabyte Poster Gold Member

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    Ouch! My head hurts... :eek:
     
  5. tripwire45
    Honorary Member

    tripwire45 Zettabyte Poster

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    Uh, how about this site:

    http://www.learntosubnet.com/

    It's all I ever needed to learn subnetting and the author of the information doesn't have his photograph posted all over the site. :wink:
     
    Certifications: A+ and Network+
  6. GiddyG

    GiddyG Terabyte Poster Gold Member

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    Heh... that is a very good site. Thanks for that; however, I have also downloaded some stuff from Paul's site and will have a gander at that as well.

    Cheers

    John
     
  7. gunga_jim

    gunga_jim Bit Poster

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    Hi there,
    The way that i would do it is:

    There are 4 octets each with 8 bits and each bit with all of the bits turned on has a maximum value of 255 therefore if you wanted to display one octet simply it would look like:

    128 192 224 240 248 252 254 255 (these numbers are gained just by adding the values below 128+64 etc)
    128 64 32 16 8 4 2 1
    1 + 1 + 1 + 1 + 1+ 1 + 1 + 1 = 255 available hosts(this is with all the bits on)

    so with this (the very last octet) the maximum number of hosts available would be 255 (including the broadcast address. But you are looking for 3000 hosts so:

    3rd OCTET 4th OCTET
    128 192 224 240 248 252 254 255 128 192 224 240 248 252 254 255
    32768 16384 8192 4096 2048 1024 512 256 128 64 32 16 8 4 2 1


    These bottom numbers represent the amount of hosts. Therefore if you need 3000 hosts you need to take 5 "bits" from the 3rd octet and the value for the subnet mask will be the number above therefore

    255.255 for the first 2 octets (as these remain intact) and what number will allow you to have 3000 hosts? 4096 (but you must minus 2 because 1 is for the network address and the other for the broadcast address leaving 4094)

    therefore 255.255.240.0
     
    Certifications: MCP & CCNA
    WIP: CCNP maybe CCVP whilst i'm there...
  8. gunga_jim

    gunga_jim Bit Poster

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    Ooooh, it all looked very nicely spaced out when I typed it... oh well, i hope you can understand it.
     
    Certifications: MCP & CCNA
    WIP: CCNP maybe CCVP whilst i'm there...
  9. ffreeloader

    ffreeloader Terabyte Poster

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    Use the
    Code:
     tags instead of the [quote] tags and your text will stay formatted.
     
    Certifications: MCSE, MCDBA, CCNA, A+
    WIP: LPIC 1

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