Subnetting **** Sheet

rob42rob42 Posts: 423Member
I've come up with a little '**** sheet' for Subneting that may be of help to others.



SUBNET
/ 25
/ 26
/ 27
/ 28
/ 29
/ 30




MASK
128
192
224
240
248
252




BLOCK
128
64
32
16
8
4




HOSTS
126
62
30
14
6
2




SUBNETS
2
4
8
16
32
64




BIT VALUE
128
64
32
16
8
4
2
1



As you can see, Blocks = Bit Value and Hosts = Block -2

Starting at 128, Mask = Mask + the next Block down.

Subnets go up in powers of 2 (Left to Right), in the same way as Bit Values go up in powers of 2 (Right to Left).

So, if you're given a list of IPs...

192.168.1.25 /29
192.168.1.28 /29
192.168.1.30 /29
192.168.1.41 /29
192.168.1.46 /29
192.168.1.55 /29
192.168.1.81 /29
192.168.1.86 /29
192.168.1.87 /29

... you can see that you have a Block Size of 8, so the Networks will be...
0, 8, 16, 24, 32, 40, 48, 56, 64, 72, 80, 88... (and so on, but we're there) with 6 Host IPs per Block.

We can now group the IPs into Networks.

Network .24
IPs .25 -> .30
B/Cast .31

192.168.1.25 /29
192.168.1.28 /29
192.168.1.30 /29

Network .40
IPs .41 -> .46
B/Cast .47

192.168.1.41 /29
192.168.1.46 /29

Network .56
IPs .57 -> .62
B/Cast .63

192.168.1.55 /29

Network .80
IPs .81 -> .86
B/Cast .87

192.168.1.81 /29
192.168.1.86 /29
192.168.1.87 /29

You can also see that the Mask will be 255.255.255.248 and the Binary will be
11111111.11111111.11111111.11111000

I hope this is of help.
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Comments

  • VeritiesVerities Posts: 1,162Member
    You're awesome. Thank you!
  • volfkhatvolfkhat Posts: 947Member ■■■■■■■□□□
    i tweaked it for you :]



    CIDR

    / 24

    / 25

    / 26

    / 27

    / 28

    / 29

    / 30

    / 31



    MASK

    .0

    .128

    .192

    .224

    .240

    .248

    .252

    .254



    BLOCK

    256

    128

    64

    32

    16

    8
    4
    2



    HOSTS

    254

    126

    62

    30

    14

    6

    2

    2



    SUBNETS

    1

    2

    4

    8

    16

    32

    64
    128



    BIT VALUE

    256
    128
    64

    32

    16

    8

    4
    2


  • rob42rob42 Posts: 423Member
    volfkhat wrote: »
    i tweaked it for you :]



    CIDR
    / 24
    / 25
    / 26
    / 27
    / 28
    / 29
    / 30
    / 31


    MASK
    .0
    .128
    .192
    .224
    .240
    .248
    .252
    .254


    BLOCK
    256
    128
    64
    32
    16
    8
    4
    2


    HOSTS
    254
    126
    62
    30
    14
    6
    2
    2


    SUBNETS
    1
    2
    4
    8
    16
    32
    64
    128


    BIT VALUE
    256
    128
    64
    32
    16
    8
    4
    2

    Hay bud'

    Either there's a gap in my knowledge or you can't have a /31?

    My understanding is that you'd only have 1 bit left for the Nodes, but a minimum of 2 bits are required.

    2 bits (2^2) = 4 IPs: 1 for the Network, 2 for Hosts, 1 for the B/Cast.
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  • rob42rob42 Posts: 423Member
    Verities wrote: »
    You're awesome. Thank you!

    You're very welcome. I'm glad that someone finds it of use.
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  • rob42rob42 Posts: 423Member
    Here's another one, for Class B Networks.



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


    MASK
    128
    192
    224
    240
    248
    252
    254
    255


    BLOCK
    128
    64
    32
    16
    8
    4
    2
    1


    HOSTS
    32766
    16382
    8190
    4094
    2046
    1022
    510
    254


    SUBNETS
    2
    4
    8
    16
    32
    64
    128
    256


    BIT VALUE
    128
    64
    32
    16
    8
    4
    2
    1




    You may want to 'tweak' it in the way that volfkhat suggests.
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  • volfkhatvolfkhat Posts: 947Member ■■■■■■■□□□
    rob42 wrote: »
    Hay bud'

    Either there's a gap in my knowledge or you can't have a /31?

    My understanding is that you'd only have 1 bit left for the Nodes, but a minimum of 2 bits are required.

    2 bits (2^2) = 4 IPs: 1 for the Network, 2 for Hosts, 1 for the B/Cast.

    Almost.
    a /31 does mean that you have 1-bit reserved for the hosts.
    So... 2 ^ 1-bit = 2

    1 address for the first device.
    1 address for the other device.

    This is called Point-to-Point :]

    (a broadcast & subnet address is Not required)
  • volfkhatvolfkhat Posts: 947Member ■■■■■■■□□□
    And if you drop the "Hosts" row....
    you will see that it's actually the same table from /8 - /31

    _______________ /8 ___ /9 ___ /10 ___ /11 ___ /12___ /13 __ /14 ___ /15
    ______________ /16 __ /17 ___ /18 ___ /19 ___ /20 ___ /21 __ /22 __ /23


    CIDR
    / 24
    / 25
    / 26
    / 27
    / 28
    / 29
    / 30
    / 31


    MASK
    .255

    .128
    .192
    .224
    .240
    .248
    .252
    .254


    BLOCK
    1

    128
    64
    32
    16
    8
    4
    2


    HOSTS










    SUBNETS
    1

    2
    4
    8
    16
    32
    64
    128


    BIT VALUE
    1

    128
    64
    32
    16
    8
    4
    2




    (although, i don't quite understand what you mean by the "bit value" row; i left it in)
  • rob42rob42 Posts: 423Member
    Cheers for the Point-to-Point info; I suspected that it was a gap in my knowledge, a gap now filled. Thank you.

    That's a good idea as /8 to /31 can all be put in the same table.

    The reason behind the Bit Value row is so that the Binary is easy to see.

    E.g: If you wanted the Binary for 224, you just but a '1' under all the columns, up to and including the '224' column, and a '0' under the remaining columns, and there you have it:- 1 1 1 0 0 0 0 0.

    That won't work if the table's set out in the way you've done it, mind.
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  • rob42rob42 Posts: 423Member
    Maybe this would be better. Putting in the number of Hosts would get very messy.



    CIDR
    (Network bits)
    /25
    /17
    /9
    /26
    /18
    /10
    /27
    /19
    /11
    /28
    /20
    /12
    /29
    /21
    /13
    /30
    /22
    /14
    /31
    /23
    /15
    ---
    /24
    /16


    MASK
    128
    192
    224
    240
    248
    252
    254
    255


    BLOCK SIZE

    128
    64
    32
    16
    8
    4
    2
    1


    HOST bits
    7
    15
    23
    6
    14
    22
    5
    13
    21
    4
    12
    20
    3
    11
    19
    2
    10
    18
    1
    9
    17
    ---
    8
    16


    BIT VALUE
    128
    64
    32
    16
    8
    4
    2
    1



    Just know that the number of IPs = 2 ^ Host Bits and Host = IPs - 2
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  • volfkhatvolfkhat Posts: 947Member ■■■■■■■□□□
    rob42 wrote: »
    E.g: If you wanted the Binary for 224, you just but a '1' under all the columns, up to and including the '224' column, and a '0' under the remaining columns, and there you have it:- 1 1 1 0 0 0 0 0.

    That won't work if the table's set out in the way you've done it, mind.

    OH man..... you are Right!
    Arranging your table from /9 - /32 really is the Logical way to go.
    lol

    Also, i kept calling the /24 mask a .0
    but, you are right again.. it's really a .255

    looks like i learned something today :]


    of course, when i teach subnetting to beginners... i think calling it ZERO is a little less intimidating.
    lol
  • rob42rob42 Posts: 423Member
    volfkhat wrote: »
    ...
    looks like i learned something today :]
    ...

    So did I. Thank you. :}

    The table is also of use for VLSMs as it's clear to see where the Boundaries fall.
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  • volfkhatvolfkhat Posts: 947Member ■■■■■■■□□□
    okay,
    let's go for some Extra credit!

    Suppose that you had a computer with a CIDR address of:
    3.3.3.3/3

    What can you tell me about the subnet it belongs to?
    :]
  • rob42rob42 Posts: 423Member
    You like to test me, don't you ;)

    With my system I'd have to add a row 1 -> 8...

    CIDR/N-Bits
    9 | 10| 11| 12| 13| 14| 15|16
    1 | 2 | 3 | 4 | 5 | 6 | 7 | 8
    Mask
    128|192|224|240|248|252|254|255
    Block Size/Bit-Value 128|64 |32 |16 | 8 | 4 | 2 |1

    Block Size = 32

    Subnets: 0, 32, 64, 96, 128 fall in the 1st Octet.

    Subnet = 0

    1st Subnet 0.0.0.0
    1st U.IP = 0.0.0.1
    Last U.IP 31.255.255.254
    B/Cast = 31.255.255.255
    Mask = 224.0.0.0

    2nd Subnet
    32.0.0.0
    etc...

    It does actually throw up an interesting question, mind. Is that a valid Network IP? I can see that in theory, it is, but in practice?
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  • rob42rob42 Posts: 423Member
    For anyone following along, this is the final draft. I've made it as easy as I can, so that it can be written out from memory. Also, the Sub-Rows now match the Octets.



    CIDR
    SUBNET /bits
    /1
    /9
    /17
    /25
    /2
    /10
    /18
    /26
    /3
    /11
    /19
    /27
    /4
    /12
    /20
    /28
    /5
    /13
    /21
    /29
    /6
    /14
    /22
    /30
    /7
    /15
    /23
    /31
    /8
    /16
    /24
    /32



    Block Size
    128
    64
    32
    16
    8
    4
    2
    1


    Mask
    128
    192
    224
    240
    248
    252
    254
    255


    HOST bits
    31
    23
    15
    7
    30
    22
    14
    6
    29
    21
    13
    5
    28
    20
    12
    4
    27
    19
    11
    3
    26
    18
    10
    2
    25
    17
    9
    1
    24
    16
    8
    0




    Start by writing out 1 2 3 4 5 6 7 8
    Then, under that, 9 -> 16, under that, 17 -> 24 and under that, 25 -> 32.

    The next Row represents both the Block Size as well as the Bit Position Value...

    128 64 32 16 8 4 2 1

    The Mask Row starts at 128; just add the Block Size to the Mask, left to right.

    (12icon_cool.gif + 64 = (192) + 32 = (224) + 16 etc...

    The Host Bits is really an option, but may be of use (= 32 - the CIDR)

    That's it.

    So, apply an IP (off the top of my head) 156.23.6.8 /18

    /18 is in Sub-Row 3 of the CIDR, so the interesting Octet is the 3rd Octet.

    Our Mask has to be 255.255.192.0

    We have a Block size of 64, so the Subnets are 0, 64, 128 (again all in the 3rd Octet).

    1st Subnet: 156.23.0.0
    2nd Subnet: 156.23.64.0

    So, the Broadcast IP for this example has to be 156.23.63.255 (one address lower than the 2nd Subnet).

    That gives us a usable Host Range of 156.23.0.1 -> 156.23.63.254

    My thanks to volfkhat for his collaboration on this and I hope that others will find it of use.
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  • volfkhatvolfkhat Posts: 947Member ■■■■■■■□□□
    Yep!
    you got it :]


    dnsstuff.com:
  • rob42rob42 Posts: 423Member
    Thanks for your input.

    Just to be clear, the Subnet for 3.3.3.3 /3 is 1.0.0.0, right? {Binary for Class A: 0.0.0.0.0.0.0.1 – 0.1.1.1.1.1.1.0}
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