# Subnetting **** Sheet

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

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

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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• Posts: 1,162Member
You're awesome. Thank you!
• Posts: 944Member ■■■■■■■□□□
i tweaked it for you :]

CIDR

/ 24

/ 25

/ 26

/ 27

/ 28

/ 29

/ 30

/ 31

.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

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

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

.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.
No longer an active member
• Posts: 423Member
Verities wrote: »
You're awesome. Thank you!

You're very welcome. I'm glad that someone finds it of use.
No longer an active member
• Posts: 423Member
Here's another one, for Class B Networks.

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

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.
No longer an active member
• Posts: 944Member ■■■■■■■□□□
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 :]

• Posts: 944Member ■■■■■■■□□□
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

.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)
• 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.
No longer an active member
• 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

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
No longer an active member
• Posts: 944Member ■■■■■■■□□□
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
• 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.
No longer an active member
• Posts: 944Member ■■■■■■■□□□
okay,
let's go for some Extra credit!

3.3.3.3/3

What can you tell me about the subnet it belongs to?
:]
• 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
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

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?
No longer an active member
• 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

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.
No longer an active member
• Posts: 944Member ■■■■■■■□□□
Yep!
you got it :]

dnsstuff.com:
• Posts: 423Member