Subnetting **** Sheet
rob42
Member Posts: 423
in CCNA & CCENT
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.
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.
No longer an active member
Comments
-
volfkhat Member Posts: 1,071 ■■■■■■■■□□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 Member Posts: 423i 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
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 -
rob42 Member Posts: 423You're awesome. Thank you!
You're very welcome. I'm glad that someone finds it of use.No longer an active member -
rob42 Member Posts: 423Here'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.No longer an active member -
volfkhat Member Posts: 1,071 ■■■■■■■■□□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 Member Posts: 1,071 ■■■■■■■■□□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 Member Posts: 423Cheers 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 -
rob42 Member Posts: 423Maybe 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 - 2No longer an active member -
volfkhat Member Posts: 1,071 ■■■■■■■■□□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 Member Posts: 423...
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 -
volfkhat Member Posts: 1,071 ■■■■■■■■□□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 Member Posts: 423You 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?No longer an active member -
rob42 Member Posts: 423For 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.No longer an active member -
rob42 Member Posts: 423Thanks 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}No longer an active member