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Subnetting: Why am I always 2 off???

Discussion in 'Network+' started by orangepeeleo, Oct 21, 2010.

  1. orangepeeleo

    orangepeeleo Nibble Poster

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    Hey guys,

    Just got to the subnetting chapter of the AIO and have been doing practice questions online re: subnetting, not having too much problems with it but there is one thing that i am consistently doing wrong and i don't know why, for example:

    Which subnet does host 172.30.99.75/21 belong to?

    The way i would work it out would be to put the subnet mask into binary so:

    11111111.11111111.11111000.00000000

    from the 3rd octet i would work out that the intervals for the subnets would be of 7 so the closest divisor of 7 to 99 would be 98, so 172.30.99.75/21 sits on 172.30.98.0..... in my crazy mind at least!

    The answer according to the site is .30.96.0 and the other similar questions i have done also resulted in me being 2 off so i must be doing something incredibly simple wrong somewhere!

    BTW forgive me if my working out is a bit messy, the AIO chapter and methods confused the hell out of me so my subnetting knowledge comes from various online sources :D
     
    Certifications: A+
    WIP: N+
  2. BosonMichael
    Highly Decorated Member Award

    BosonMichael Yottabyte Poster

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    Subnet intervals are always powers of 2. In this case, it'd be 8...

    0 to 7
    8 to 15
    16 to 23...

    ...etc.

    I think you're seeing the 7 and assuming that there's 7 digits from 1 to 7... but you're forgetting 0.

    In this scenario, the closest multiple of 8 (without going over) is 96.

    :thumbleft
     
    Certifications: CISSP, MCSE+I, MCSE: Security, MCSE: Messaging, MCDST, MCDBA, MCTS, OCP, CCNP, CCDP, CCNA Security, CCNA Voice, CNE, SCSA, Security+, Linux+, Server+, Network+, A+
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  3. bazzawood30

    bazzawood30 Byte Poster

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    use your binary chart

    3rd oct
    The numbers line up in my text box but not when i publish it so sorry for the binary chart not displaying correctly
    128 64 32 16 8 4 2 1
    1 1 1 1 1 0 0 0

    look whats above your last 1 of the subnet mask

    or 256-248=8

    not 255 as boson 0 is a value

    Jumps would be
    0-7
    8-15
    16-23
    24-31
    32-39
    40-47
    48-55
    56-63
    64-71
    72-79
    80-87
    88-95
    96-103
    104-111
    and so on
     
    Last edited: Oct 21, 2010
    Certifications: ECDL,A+,N+,CCENT,CCNA,MCP,MCDST
  4. greenbrucelee
    Highly Decorated Member Award

    greenbrucelee Zettabyte Poster

    14,283
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    If you make a chart like this:-

    128 64 32 16 8 4 2 1 0

    This can help you when working out the whole subnet

    so say the IP and subnetmask is 192.168.1.0/24 or 255.255.255.0

    You have 1 IP address and you have 4 networks with the subnet mask given we can only have 1 network so you need to steal some bits and make the subnetmask into 255.255.255.192 which /26 this will give us the 4 networks.

    so to work out the subnet (if you do this a few times it will start to sink in). You need to steal some bits from one side and give to the other so if you use your binary chart you have the IP

    192.168.1.0 = 11000000.10101000.00000001.00000000
    255.255.255.192 = 11111111.11111111.11111111.11000000 (the two ones in the final octect are the subnet leaving 6 bits which is 62

    so using the chart that gives us 62 hosts and 4 networks.

    Hopefully that makes a bit of sense subnetting is a bit confusing when you first start to do, I'm still not good at it yet.
     
    Last edited: Oct 21, 2010
    Certifications: A+, N+, MCDST, Security+, 70-270
    WIP: 70-620 or 70-680?
  5. DC Pr0Mo

    DC Pr0Mo Kilobyte Poster

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    This

    Looks to me as well that you're looking at the 3 zeros at the end of 11111000 and seeing 7 different combinations rather than eight.

    000
    001
    010
    011
    100
    101
    110
    111

    I suppose you could always take the subnet mask binary number 11111000 convert it to decimal 248, then subtract 248 from 256, this will give you the correct number of intervals in the octect.
     
    Certifications: MCDST | BSc Network Computing
    WIP: 70-291 | 70-293 | 70-294 | 70-297

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