# Help With Breaking Down Subnetting

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I get the basic concept of it, but I cannot seem to wrap my head around the actual math of it.

I can (kinda) do class C addresses, but I'm getting stuck on how to tell how many subnets/hosts per subnet are there if given just the mask.

Example: Find the number of subnets for a mask of 255.255.255.224.

Edited by Charles
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"I get the basic concept of it, but I cannot seem to wrap my head around the actual math of it."

This might help if your more of a visual learner... may I present "The ????? Box"

_____________________________________________________________________________

***The ????? Box is a pain to display without tab formatting. You probably want to copy and paste this stuff into notepad or whatever to add tabs where the "|" are. The "||" are meant to signify your finger... or something.***

_____________________________________________________________________________

_______________________________________________________________________

| 128 | 192 | 224 | 240 | 248 | 252 | 254 | 255 | [subnet Mask]

|______________________________________________________________________

| 128 | 64 | 32 | 16 | 8 | 4 | 2 | 1 | [Where the Binary Bit = #]

|______________________________________________________________________

| 255 | 127 | 63 | 31 | 15 | 7 | 3 | 1 | [Network]

_______________________________________________________________________

(+1 for nets) (-1 for hosts)

... and for CIDR notation, replace [Network] w/ the row below ...

__________________________________________________________________________

| /25 | /26 | /27 | /28 | /29 | /30 | /31 | /32 | [CIDR Notation ]

__________________________________________________________________________

Using the visual "graph" above...

"... Find the number of subnets for a mask of 255.255.255.224."

Everything to the RIGHT of your finger is AVAILABLE (eg. subnet-able?)

||

_____________________________________

~MASK | 192 | 224 || 240 |

_________|___________________________

~BINARY | 64 | 32 || 16 |

_________|___________________________

~NETWRK| 127 | 63 || 31 | (-1 for hosts / +1 for nets)

_____________________________________

||

Step 2) Moving from RIGHT to LEFT, how many bits do you have to count to reach your finger?

||

__________________________________________________________________________

~MASK | 192 | 224 || 240 | 248 | 252 | 254 | 255 |

_________|________________________________________________________________

~BINARY | 64 | 32 || 16 | 8 | 4 | 2 | 1 |

_________|________________________________________________________________

~NETWRK | 127 | 63 || 31 | 15 | 7 | 3 | 1 | (-1 hosts / +1 nets)

__________________________________________________________________________

||

Step 3) Reverse the process, moving from LEFT to RIGHT, count the same amount of bits to find the number of subnets for 255.255.255.224.

||

__________________________________________________________________________

~MASK | 128 | 192 | 224 || 240 | 248 | 252 | 254 |

__________________________________________________________________________

~BINARY| 128 | 64 | 32 || 16 | 8 | 4 | 2 |

__________________________________________________________________________

~NETWRK| 255 | 127 | 63 || 31 | 15 | 7 | 3 | (-1 hosts / +1 nets)

__________________________________________________________________________

||

Not only can I tell you the total amount of subnets for a Subnet Mask of 255.255.255.224, but I can also tell you that...

- there is 30 hosts (eg. IP Addresses) per subnet [ technically 32, but 1 is needed for the NET ID & another 1 is needed for the BROADCAST ID ]

- the CIDR notation for this would ?.?.?.?/27 [ where the ?'s are the NET ID of the network (eg. 192.168.0.0) and the amount of RESERVED (or MASKED) bits (27)]

- there are 3 subnet bits [ total bits in an IPv4 address: 32. Amount of bits RESERVED in THIS Octet = 3]

- there are 27 masked bits [ total bits in an IPv4 address: 32. Amount of bits RESERVED = 27]

Using "The ????? Box" for Class A and B addresses is possible. What youd be doing is adding more information on the LEFT side of "The ????? Box"

The MASK value will start over again @ 255 [ because this is now a new octet we're dealing with]

The BINARY Row will continue to multiply by 2 [ 256, 512, 1024, etc... ]

The NETWRK Row will continue to be the SUM of the CURRENT BINARY Column and the PREVIOUS NETWRK column. [ So if the BINARY column says "256", and the previous NETWRK column was "255", then our new NETWRK column will be "511"]

If your not a visual learner, then you may do better with the following...

11111111.11111111.11111111.11100000

___________________________________

Total amount of 1's = 27[ /27 is our CIDR notation ]

Total amount of 0's = 5 [ The amount of Bits AVAILABLE]

Total amount of 1's (in this octet) = 3 [ The amount of our Subnet bits (eg. RESERVED)]

If we "flipped" all of our 0's to 1's our Binary value would have increased by 31 [because each Bit represents a value, (eg. Last bit = 1, 2nd to last bit = 2, 3rd to last bit = 4, etc...]

If we subtract 31 from 255 we get 224 (our subnet mask for this octet).

If we divide 31 into 255 we get 8*, which is the total number of subnets for this octet. [*actually we should add 1 to 31 & 255 to get the correct network number]

Again, we need to note that 1 IP address will be our NET ID and another with be our BROADCAST ID. So 32 minus, 2 leaves us with 30 hosts per subnet.

When this stuff gets pretty easy when you have that A-HA! moment. Hope you got it, or got some sleep from reading this.

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I get the basic concept of it, but I cannot seem to wrap my head around the actual math of it.

I can (kinda) do class C addresses, but I'm getting stuck on how to tell how many subnets/hosts per subnet are there if given just the mask.

Example: Find the number of subnets for a mask of 255.255.255.224.

Subnetting was a pain for me to learn, but I have a little cheat sheet I used to help me:

Might not make sense to you when looking at it, but you need to learn your binary math.

Borrowed Bits + Default Subnet Mask Bits = CIDR Value.

Example: 11111111 11111111 11000000 00000000

CIDR Value: /18

For every bit you add to the default mask, you are subnetting. Every bit you remove, supernetting.

Its 2 to the power of every bit you remove, or for 2 to the power of every bit borrowed, this is your subnet mask. For every (2 to the power of the remaining bit to the right)-2, that is your number of hosts.

I used to practice doing subnets on a regular basis for my cisco class, but you can also verify you did them correctly by checking them against this: http://www.subnet-calculator.com/

Edited by digip
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Thanks for the explaination. I did custom subnetting a few years ago for a class but haven't used it since. It is hard to remember something if you don't use it. Heh.

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I work in a blue chip IT firm as a consultant, and my dirty secret is that I still need the subnetting application on my cell phone.

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Too bad you aren't allowed to use subnetting calculators on exams!

I'd probably have one on my phone or netbook if I needed to do it alot.

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my dirty secret is that I still need the subnetting application on my cell phone.

Do you use the Subnet Calc app or something else?

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Subnetting can be quite confusing sometimes. I always use a Subnet calculator for doing the math.

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I use IPcalc.

Do you use the Subnet Calc app or something else?

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