Subnetting **** Sheet

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

Verities

You're awesome. Thank you!

volfkhat

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

rob42

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.

rob42

Verities wrote: »

You're awesome. Thank you!

You're very welcome. I'm glad that someone finds it of use.

rob42

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.

volfkhat

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)

volfkhat

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)

rob42

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.

rob42

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

volfkhat

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

rob42

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.

volfkhat

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?
:]

rob42

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?

rob42

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.

(12

+ 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.

volfkhat

Yep!
you got it :]

dnsstuff.com:

dnsstuf.jpg

rob42

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}