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Subnetting Question

Discussion in 'Routing & Switching' started by kammodo, Feb 12, 2008.

  1. kammodo

    kammodo Nibble Poster

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    Hello, just opened the can of evil worms which is subnetting,
    My question is this:

    IP address 10.0.0.0 (class A)
    Mask 255.255.240.0
    Valid subnets 256-240=16
    Subnets 4094
    Hosts 4094
    What would be the first valid Subnet address be? A) 10.0.16.0 - 10.0.32.0 - 10.0.48.0 ect or B) 10.16.0.0 – 10.32.0.0

    Is the address set from left to right as in B or the other way around? Is there a rule of the thumb on this, or dosen’t it matter as they should have the same amount
    of hosts and broadcasts on each?
    Any help much appreciated
    Thanks
     
    Certifications: CCNA R+S, CCDA , CCNP r+s , CCDP
    WIP: CCIE
  2. hbroomhall

    hbroomhall Petabyte Poster Gold Member

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    If you are talking classes then the rules change.
    Eh? You have just said 16!
    Counting how?
    Depends on how the router is set. Cisco allows 0 nets by default these days I believe, so neither is right. But A is closer. First IP is 10.0.0.0.
    Don't understand this at all. There can never be the same number of hosts and broadcast addresses on a network!

    Harry.
     
    Certifications: ECDL A+ Network+ i-Net+
    WIP: Server+
  3. Mr.Cheeks

    Mr.Cheeks 1st ever Gold Member! Gold Member

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    Struggling on Subnetting? Check this out!
     
  4. kammodo

    kammodo Nibble Poster

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    Sorry, I didn't explain myself properly.
    I know you more than likley already know these basic caculations but these are my working outs for the problem.

    Ip address 10.0.0.0
    Mask 255.255.240.0

    Bits used for subnets 12
    Bits left over for host protion 12
    Total avail subnets, 2 to the power of 12 -2 = 4094
    Total avail hosts, 2 to the power of 12 -2 = 4094
    Seperation between each subnet or first sub net ID 256-240=16 Which is my question, on what is the first subnet IP address? A) 10.0.16.0 - 10.0.32.0 - 10.0.48.0 ect or B) 10.16.0.0 – 10.32.0.0 -10.48.0.0 ect.
    I understand that the first IP address is 10.0.0.0 the one that the rest of the world sees(router), its just the internal subnet IDs that I am having trouble with.
    At the end I ment to say host numbers are the same as the subnet numbers not broadcast numbers which are obviously different (doh!).
    Thanks for the help
     
    Certifications: CCNA R+S, CCDA , CCNP r+s , CCDP
    WIP: CCIE
  5. BrotherBill

    BrotherBill Byte Poster

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    Your mask is 255.255.240.0. You still have to count the 0 in the second octate. Use 10.0.x.x
     
  6. kammodo

    kammodo Nibble Poster

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    Yes that does make sense.
    Thanks for the reply
     
    Certifications: CCNA R+S, CCDA , CCNP r+s , CCDP
    WIP: CCIE
  7. BrotherBill

    BrotherBill Byte Poster

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    Even though you're working with a class A address, you still have to consider the entire mask for your subnet addresses.

    I agree with Mr. Cheeks. Be sure to check out the tutorials he pointed out. You'll find them a great learning tool. http://www.learntosubnet.com/
     
  8. kevicho

    kevicho Gigabyte Poster

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    It Should be a, as it will run from 10.0.0.0 which is the network address to 10.0.15.255 which is the broadcast address for the first subnet.

    You always use the first octet which is not full (ie 255) to set the octet to be used for dividing the subnets (im sure that sounded easier in my head to explain lol), thus here 255.255.240.0 has the 3rd octet used for the host/subnet octet, as we see below

    table for this network
    1) 10.0.0.0 - 10.0.15.255
    2) 10.0.16.0 - 10.0.31.255
    3) 10.0.32.0 - 10.0.47.255
    4) 10.0.48.0 - 10.0.65.255

    and so on

    Subnetting gets easier as you read through it, also you need to remember to stop at how many subnets you create, for example if you switch from a 19 to a 21 cidr network, then you have created 4 subnets, which means that 66.0 - 73.255 is outside the range.

    The microsoft press book is great for this section
     
    Certifications: A+, Net+, MCSA Server 2003, 2008, Windows XP & 7 , ITIL V3 Foundation
    WIP: CCNA Renewal
  9. kammodo

    kammodo Nibble Poster

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    Thankyou all for the help. Im sure ill be pulling my hair out, on the next ccna chapter.:cry:
     
    Certifications: CCNA R+S, CCDA , CCNP r+s , CCDP
    WIP: CCIE

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