Fundamentals of Classful IPv4 addressing

    Author:Navneet Gaur
    Date:August 2011
    Description:The following document explains the way Classful IPv4 address range has been created
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    This is an additional explanation to the table on Classful addressing in exam certification guide, Pg 110, ICND1, by Wendell Odom

     

    The table is as follows:

     

    ClassFirst Octet Range
    Valid Network Numbers

    Total number for

    this class of network

    Number of

    hosts per network

    A1 to 1261.0.0.0 to 126.0.0.02^7 - 2* (126)2^24 - 2 (16,777,214)
    B128 to 191128.0.0.0 to 191.255.0.02^14 (16,384)2^16 - 2 (65,534)
    C192 to 223192.0.0.0 to 223.255.255.02^21 (2,097,152)2^8 - 2 (254)

     

    - 2* This is to subtract the first network number 0.0.0.0 and last network 127.0.0.0. Out of this 127.0.0.0 is reserved for loopback address, which can be assigned to loopback adapters.

     

    The portion colored in blue is explained in the following paragraphs.

     

    Point 1

    The network number part of the bits is defined using the first octet.

     

    Point 2

    The bits that will be used to define the network number are determined by the default mask for that class.

     

    Accordingly,

     

    Explanation 1

     

    Class A has a default mask of 8 bits that translates to 255.0.0.0 in decimal as follows.

     

    Class A

    1st Octet


    1st eight bits

    out of 32 bits

    2nd Octet

    Bits numbered

    9 to 16

    from the left

    3rd Octet

    Bits numbered

    17 to 24

    from the left

    4th Octet

    Bits numbered

    25 to 32

    from the left

    Role of the bits as 'dictated

    by the mask'

    underneath the IP address

    Network Bits

    Host bits

    Bits used for

    numbering hosts

    Host bits

    Bits used for

    numbering hosts

    Host bits

    Bits used for

    numbering hosts

    nnnnnnnnhhhhhhhhhhhhhhhhhhhhhhhh
    Ip Address*'0'0000000000000000000000000000000

    Default network mask

    for "Class A" address'

    11111111000000000000000000000000
    Mask in Decimal255000

     

    First octet is used to decide the range of network address' available for a particular class of Ip address'.

     

    The default network mask dictates the total number of bits available for all network numbers in that range.

     

    Therefore in 'class A' first 8 bits are used for network ranges, as well as the fact that the default mask dictates that first eight bits are all that would indicate the network identity.

     

    Class A -   *0xxxxxxx

     

    First octet is used for numbering the networks as defined by default mask, first octet is also used to decide the range.

     

    Available number of bits that can be 'modified' as represented by 'x' are 7 and therefore number of possible different network number combinations using those bits is equal to 2^7

     

    The first bit - represented by '0', stays the same for all the combinations.

     

    So the first combination is - '0' 0000000 = decimal 0

     

    Then -   '0' 0000001 = decimal 1

    Then -   '0' 0000010 = decimal 2

    Finally - '0' 1111111   = decimal 127

     

    Bits that can be changed are colored red.

    Total combinations using the bits that can be changed - 2^7

     

    Therefore the range of Class A ip address starts from 0 and ends at 127

     

     

    Explanation 2


    Class B has a default mask of 16 bits that translates to 255.255.0.0 in decimal as follows.

     

    Class B

    1st Octet


    1st eight bits

    out of 32 bits

    2nd Octet

    Bits numbered

    9 to 16

    from the left

    3rd Octet

    Bits numbered

    17 to 24

    from the left

    4th Octet

    Bits numbered

    25 to 32

    from the left

    Role of the bits as

    'dictated
     
    by the mask'

    underneath the IP address

    Network BitsNetwork Bits

    Host bits

    Bits used for

    numbering hosts

    Host bits

    Bits used for

    numbering hosts


    nnnnnnnnnnnnnnnnhhhhhhhhhhhhhhhh
    Ip Address*'10'000000*000000000000000000000000

    Default network mask

    for "Class B" address'

    11111111111111110000000000000000
    Mask in Decimal25525500

     

    First octet is used to decide the range of network address' available for a particular class of Ip address'.

     

    The default network mask dictates the total number of bits available for all network numbers in that range.

     

    Therefore in 'class B' first 16 bits are used for numbering the networks, as the default mask dictates that first sixteen bits should indicate the network identity.

     

    Class B - 10xxxxxx.xxxxxxxx

     

    First two octets are used for numbering the networks as defined by default mask, first octet is used to decide the range

     

    Available number of bits that can be 'modified' as represented by 'x' are 14 and therefore number of possible different network number combinations using those bits is equal to 2^14

     

    The first two bits - represented by '10', stay the same for all the combinations.

     

    So the first combination is - '10' 000000.00000000 = decimal 128.0

     

    Then -   '10' 000000.00000001 = decimal 128.1

    Then -   '10' 000000.00000010 = decimal 128.2

    Finally - '10' 111111.11111111 = decimal 191.255

     

    Bits that can be changed are colored red.

    Total combinations using the bits that can be changed - 2^14

     

    Therefore the range of Class B ip address starts from 128 and ends at 191

     

     

    Explanation 3


    Class C has a default mask of 24 bits that translates to 255.255.255.0 in decimal as follows.

     

    Class C

    1st Octet


    1st eight bits

    out of 32 bits

    2nd Octet

    Bits numbered

    9 to 16

    from the left

    3rd Octet

    Bits numbered

    17 to 24

    from the left

    4th Octet

    Bits numbered

    25 to 32

    from the left

    Role of the bits as

    'dictated
    by the mask'

    underneath the IP address

    Network BitsNetwork BitsNetwork Bits

    Host bits

    Bits used for

    numbering hosts


    nnnnnnnnnnnnnnnnnnnnnnnnhhhhhhhh
    Ip Address*'110'00000*00000000*0000000000000000

    Default network mask

    for "Class C" address'

    11111111111111111111111100000000
    Mask in Decimal2552552550

     

    First octet is used to decide the range of network address' available for a particular class of Ip address'.

     

    The default network mask dictates the total number of bits available for all network numbers in that range.

     

    Therefore in 'class C' first 24 bits are used for network ranges, as the default mask dictates that first twenty-four bits should indicate the network identity.

     

    Class C - 110xxxxx.xxxxxxxx.xxxxxxxx

     

    First three octets are used for numbering the networks as defined by default mask, first octet is used to decide the range

     

    Available number of bits that can be 'modified' as represented by 'x' are 21 and therefore number of possible different network number combinations using those bits is equal to 2^21

     

    The first three bits - represented by '110', stay the same for all the combinations.

     

    So the first combination is - '110' 00000.00000000.00000000 = decimal 192.0.0

     

    Then -   '110' 00000.00000000.00000001 = decimal 192.0.1

    Then -   '110' 00000.00000000.00000010 = decimal 192.0.2

    Finally - '110' 11111.11111111.11111111 = decimal 223.255.255

     

     

    Bits that can be changed are colored red.

    Total combinations using the bits that can be changed - 2^21

     

    Therefore the range of Class C ip address starts from 192 and ends at 223

     

    • While in the hosts part all the bits can be varied or used with a different value if required at one time or another

     

    • Essentially, we have to understand that computers operate in binary and treat the whole string of 32 bits as one string

     

    • We have broken it up in four parts so that we can easily deal with it

     

     

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    The End