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Here is the chart that I always write before a Cisco exam while it's doing its demo. The order of the rows is random and non-essential.

 

  1          2        3        4        5        6        7        8

  7          6        5        4        3        2        1        0

IIIIIIII  IIIIIIII  IIIIIII  IIIIIIII  IIIIIIII  IIIIIIII  IIIIIII  IIIIIIII

I          II         III      IIII      IIIII     IIIIII    IIIIII   IIIIIIII

128    192     224    240    248     252    254    255

80      C0       E0      F0      F8       FC     FE     FF

/9      /10      /11     /12     /13      /14     /15    /16

/17    /18      /19     /20     /21     /22     /23     /24

/25    /26      /27     /28     /29     /30     /31      /32   

128    64      32       16        8         4        2         1

127    63      31       15        7         3        1         0

126    62      30       14       6         2        0         0

 

Explained Line by Line:

  1          2        3        4        5        6        7        8

[Subnet bits borrowed, counting from the left and including the leftmost bit position in the count]

 

  7          6        5        4        3        2        1        0    

[Host bits borrowed, counting from the right and NOT including the rightmost bit position in the count]

 

IIIIIIII  IIIIIIII  IIIIIII  IIIIIIII  IIIIIIII  IIIIIIII  IIIIIII  IIIIIIII

[Visualization of a full 32-bit subnet mask]

 

I          II         III      IIII      IIIII     IIIIII    IIIIII   IIIIIIII

[Subnet mask bits being used for the active octet]

 

128    192     224    240    248     252    254    255

[Decimal sum of the namespace for IP addresses; also the network address of the last subnet]

 

80      C0       E0      F0      F8       FC     FE     FF

[Hexadecimal sum of the namespace for IP addresses]

 

/9      /10      /11     /12     /13      /14     /15    /16

/17    /18      /19     /20     /21     /22     /23     /24

/25    /26      /27     /28     /29     /30     /31      /32   

[CIDR (Classless Inter-Domain Routing)]  

 

128    64      32       16        8         4        2         1

[Number of valid IP addresses given subnet length - known as the block size or the increment] 

 

127    63      31       15        7         3        1         0

[Wildcard IP addresses]

 

126    60      30       14        6         2        0         0

[Number of valid hosts give the subnet length, making room for broadcast and default gateway]

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My personal rant: There are not many ways to subnet. This is an extremely popular misconception. There is just the way there is. Ultimately, once you understand subnetting and can visualize how much space there is across a network topology diagram, you will be able to see the subnet masks and have access to this understanding without calculation.

Subnets can be thought of as visualizations of how much space there is. This space can be used to store data or numbers.

There is still only one way to subnet.

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