This topic works hand in hand with IPv4.
Subnet consists of 32bits in binary form, and usually written in dot-decimal notation (eg. 255.255.255.0)
Sample Calculation #1
Prefix
/24
What does it mean in binary?
Starting from left most side, there are 24 ones, as shown below
11111111.11111111.11111111.00000000
How it is written in dot-decimal?
| 11111111 |
. |
11111111 |
. |
11111111 |
. |
00000000 |
| 2^7 + 2^6 + 2^5 + 2^4 + 2^3 + 2^2 + 2^1 + 2^0
= 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1
= 255 |
|
|
|
|
|
|
| 255 |
. |
255 |
. |
255 |
. |
0 |
What is the total range?
2^( Default bits 32 – prefix 24 )
= 2^8
= 256
For the total range, how many hosts can it have? <total range> – 2
256 – 2
= 254
Sample Calculation #2
Prefix
/20
What does it mean in binary?
Starting from left most side, there are 20 ones, as shown below
11111111.11111111.11110000.00000000
How it is written in dot-decimal?
| 11111111 |
. |
11111111 |
. |
11110000 |
. |
00000000 |
|
|
|
|
|
2^7 + 2^6 + 2^5 + 2^4
= 128 + 64 + 32 + 16
= 240 |
|
|
| 255 |
. |
255 |
. |
240 |
. |
0 |
What is the total range?
2^( Default bits 32 – prefix 20)
= 2^12
= 4096
For the total range, how many hosts can it have? <total range> – 2
4096 – 2
= 4094
Subnet Table
| Prefix |
Binary Form |
Dot-Decimal Notation |
Total range |
Total hosts |
| /14 |
11111111.11111100.00000000.0000000 |
255.252.0.0 |
262,144 |
262,142 |
| /15 |
11111111.11111110.00000000.0000000 |
255.254.0.0 |
131,072 |
131,070 |
| /16 |
11111111.11111111.00000000.0000000 |
255.255.0.0 |
65,536 |
65,534 |
| /17 |
11111111.11111111.10000000.0000000 |
255.255.128.0 |
32,768 |
32,766 |
| /18 |
11111111.11111111.11000000.0000000 |
255.255.192.0 |
16,384 |
16,382 |
| /19 |
11111111.11111111.11100000.0000000 |
255.255.224.0 |
8,192 |
8,190 |
| /20 |
11111111.11111111.11110000.0000000 |
255.255.240.0 |
4,096 |
4,094 |
| /21 |
11111111.11111111.11111000.0000000 |
255.255.248.0 |
2,048 |
2,046 |
| /22 |
11111111.11111111.11111100.0000000 |
255.255.252.0 |
1,024 |
1,022 |
| /23 |
11111111.11111111.11111110.0000000 |
255.255.254.0 |
512 |
510 |
| /24 |
11111111.11111111.11111111.00000000 |
255.255.255.0 |
256 |
254 |
| /25 |
11111111.11111111.11111111.10000000 |
255.255.255.128 |
128 |
128 |
| /26 |
11111111.11111111.11111111.11000000 |
255.255.255.192 |
64 |
62 |
| /27 |
11111111.11111111.11111111.11100000 |
255.255.255.224 |
32 |
30 |
| /28 |
11111111.11111111.11111111.11110000 |
255.255.255.240 |
16 |
14 |
| /29 |
11111111.11111111.11111111.11111000 |
255.255.255.248 |
8 |
6 |
| /30 |
11111111.11111111.11111111.11111100 |
255.255.255.252 |
4 |
2 |
| /31 |
11111111.11111111.11111111.11111110 |
255.255.255.254 |
2 |
0 |
| /32 |
11111111.11111111.11111111.11111111 |
255.255.255.255 |
1 |
1 |