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What is the calculation for converting an IP address to CIDR?

    Domanda

  • Hello,

    My MCTS book explains about "CIDR" ("Classless Inter-Domain Routing"), thus:

    For example "/24".

    The "CIDR" form shows the number of "bits" turned on in the "Subnet Mask":

    255.255.224.0 is:

    11111111.11111111.11100000.00000000, equalling 19 bits turned on, or "/19".

    Unfortunately the book does not explain how you do the calculation.

    I understand that 255 adds up to 8 turned-on bits.

    However, what is the calculation that I need to know in order to convert that 224 figure into the 3 turned-on bits?

    Robert339

    Ps. I have tried inserting carrier returns to make spaces between the paragraphs in this message, but they are not saved.





    • Modificato Robert339 sabato 17 marzo 2012 21:08
    sabato 17 marzo 2012 21:01

Risposte

  • Everyone has different way of thinking but for exam purposes a table is a good way to go.

    Just keep adding from left to right and ask a question if the current sum is bigger then 224 if it is then write down 0 and if it is not write down 1.

    bits 8 7 6 5 4 3 2 1
    value 128 64 32 16 8 4 2 1 255
    binary 1 1 1 0 0 0 0 0
    128 64 32 0 0 0 0 0 224

    • Modificato Brano Lukic lunedì 19 marzo 2012 04:40
    • Contrassegnato come risposta Robert339 lunedì 19 marzo 2012 18:49
    lunedì 19 marzo 2012 04:29

Tutte le risposte

  • 224 is equivalent to 128+64+32= 2^7+2^6+2^5 and 11100000 in binary.

    19 is number of ones from left to right (8+8+3) that mask first 19 digits of IP address and the remnant digits are not taken into count because of multiplication by zeros.

    More on this subject see http://tools.ietf.org/html/rfc4632 

    Regards

    Milos

    domenica 18 marzo 2012 19:27
  •  

    Hello Milos,

    Thank you for the information. You say the "224" is equivalent to 128+64+32, but I am afraid I am missing something here. How did you calculate those figures?

    What I am looking for is the method for answering the following example question in an examination:

     

    What is the CIDR equivalent for 255.255.255.224?

    (MCTS 70:680 Assessment Test - Windows Configuration Study Guide by William Panek.)

    Regards,

    Robert339


    • Modificato Robert339 domenica 18 marzo 2012 23:16
    domenica 18 marzo 2012 23:13
  • Everyone has different way of thinking but for exam purposes a table is a good way to go.

    Just keep adding from left to right and ask a question if the current sum is bigger then 224 if it is then write down 0 and if it is not write down 1.

    bits 8 7 6 5 4 3 2 1
    value 128 64 32 16 8 4 2 1 255
    binary 1 1 1 0 0 0 0 0
    128 64 32 0 0 0 0 0 224

    • Modificato Brano Lukic lunedì 19 marzo 2012 04:40
    • Contrassegnato come risposta Robert339 lunedì 19 marzo 2012 18:49
    lunedì 19 marzo 2012 04:29
  • Hello Brano,

    Thank you. I hope I have understood how this must be done now. In case this may assist anyone else , this is how I assume we have to proceed:

     

    What is the CIDR equivalent for 255.255.255.224?

     

    bits

    8

    7

    6

    5

    4

    3

    2

    1

    binary

    1

    1

    1

    1

    1

    1

    1

    1

    value

    128

    64

    32

    16

    8

    4

    2

    1

    255

    binary

    1

    1

    1

    0

    0

    0

    0

    0

    value

    128

    64

    32

    0

    0

    0

    0

    0

    224

    To answer the above question in an examination it is necessary to learn the “Value” figures shown above, and then use them as follows:

    An Octet of 255 value, always amounts to 8 bits.

    So of our example subnet mask 255.255.255.224

    We know that

    255.255.255

    converts to:

    8 + 8 + 8 = 24 (24 bits).

    This leaves (in this example) an Octet containing “224” value, requiring conversion as follows:

    From your Table, keep adding the highest Values to each other (left to right).

    128 + 64 + 32 = 224.

    The number of “Value” figures you needed to add together to reach 224, was 3 (bits).

    Now add these 3 bits to those of the rest of the subnet mask address (which was 24), to reach your CIDR total, thus:

    24 + 3 = 27

    255.255.255.224

    = 8 + 8 + 8 + 3 = 27

    CIDR = /27


    • Modificato Robert339 lunedì 19 marzo 2012 18:47
    lunedì 19 marzo 2012 18:46