How to Create Your Own Mental Subnet Calculator

Written by sasha maggio Google
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How to Create Your Own Mental Subnet Calculator
Once the subnet calculator methods are memorised, simple notes on scratch paper will be all the test taker needs. (calculatrice image by Noé Rouxel from Fotolia.com)

Before creating mental subnet calculators, which are used to answer IP-related questions, it is necessary to understand basic IP addressing and binary counting. Mental subnet calculators help answer IP-related questions quickly, as Tony Gibbs says, "in as little as three to 15 seconds." When testing for CISSP or other Cisco certifications, spending three or more minutes on each question and writing out all of the math will likely be detrimental to the test taker's score. Mental subnet calculators help locate answers without all the math and in record time.

Skill level:
Easy

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Things you need

  • IP addressing basics
  • Binary counting basics
  • Pencil and paper

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Instructions

  1. 1

    Make a chart of three columns, each with space for seven items. Label the columns "Y0" (yes to subnet zero), "Hosts & N0" (no to subnet zero) and "Bits." Under Bits, add the items one through seven. Under Y0, start with the number two and then bring the items to 128 by multiplying each value by two: 2, 4, 8, 16, 32, 64, 128. Now, for Hosts & N0, subtract two from each item in the Y0 column to get the corresponding N0 value: 0, 2, 6, 14, 30, 62, 126.

    The chart should look like this:

    Y0: 2 4 8 16 32 64 128

    Hosts & N0: 0 2 6 14 30 62 126

    Bits: 1 2 3 4 5 6 7

  2. 2

    Make a second chart with four columns, each with space for eight items. Label the columns "Host Bits," "Network Bits," "DDN" (Dot-Decimal Notation)" and "Chunks." Host Bits and Network Bits both use the numbers one through eight, but they are listed in the reverse order:

    Host Bits: 8 7 6 5 4 3 2 1

    Network Bits: 1 2 3 4 5 6 7 8

    The Chunks values start with 128, corresponding to 8 Host Bits and 1 Network Bit, with each item divided by two until the Chunk value is one:

    Host Bits: 8 7 6 5 4 3 2 1

    Network Bits: 1 2 3 4 5 6 7 8

    Chunks: 128 64 32 16 8 4 2 1

    The DDN items also begin with 128, then add the next Chunk value to get the next DDN value:

    DDN: 128 192 224 240 248 252 254 255

    Chunks: 128 64 32 16 8 4 2 1

    128 + 64 = 192 + 32 = 224 + 16 = 240, etc.

    The finished chart should look like this:

    Host Bits: 8 7 6 5 4 3 2 1

    Network Bits: 1 2 3 4 5 6 7 8

    DDN: 128 192 224 240 248 252 254 255

    Chunks: 128 64 32 16 8 4 2 1

  3. 3

    Use these two charts to memorise value associations for answering IP-related questions quickly and efficiently. Tony Gibbs offers the sample question:

    How many bits are required to create 15 equal subnets from 192.168.1.0/24 ?

    Answer:

    /24 is the subnet mask

    The DDN for 192.168.1.0 = 255.255.255.0

    If the answer does not specify whether subnet zero is or is not allowed, look at both columns Y0 and N0. The value will be greater than or equal to 15, since the question asks for 15 equal subnets. Refer to the first chart from step one; for column Y0, 16 is the closest value that is greater than or equal to 15. Follow 16 from Y0 to the Bits column and note that 16 requires 4 bits.

    Thus, 4 additional bits are required to make 15 equal subnets, and the new subnet mask is /24 + 4 = /28, giving 192.168.1.x/28.

    If the question specifies no subnet zero may be used, look at the Hosts & N0 column and find the value greater than or equal to 15; in this case, it would be 30, which requires an additional 5 bits. Thus, /24 + 5 = /29, giving 192.168.1.x/29.

Tips and warnings

  • Binary counting is more than simply 1s and 0s. To make the most out of mental subnet calculators, have a thorough understanding of binary counting and IP addressing before moving on.

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