Private address classes

| class | from | to | CIDR notation | subnet mask |
|:-----:|:----:|:--:|:-------------:|:-----------:|
| A | 10.0.0.0 | 10.255.255.255 | /8 | 255.0.0.0 |
| B | 172.16.0.0 | 172.31.255.255 | /12 | 255.240.0.0 |
| C | 192.168.0.0 | 192.168.255.255 | /16 | 255.250.0.0 |
| APIPA | 169.254.0.0 | 169.254.255.255 | /16 | 255.250.0.0 |

---

CIDR notation rules

* subnet mask tells what portion of the address is
  - network part (255)
  - host part (0)

```
2^0 = 1
2^1 = 2
2^2 = 4
2^3 = 8
2^4 = 16
2^5 = 32
2^6 = 64
2^7 = 128
```
---

```
/25 = 8+8+8+1
255       255       255       128
11111111  11111111  11111111  10000000

N = first value after last 255 e.g. 128
network increment:    256 - N = 256 - 128 = 128
number of subnets:    256 / 128 = 2
first valid address:  network id + 1
last valid address:   next network id - 2
```

| network id | first valid address | last valid address | broadcast address |
|:----------:|:-------------------:|:------------------:|:-----------------:|
| x.x.x.0 | x.x.x.1 | x.x.x.126 | x.x.x.127 |
| x.x.x.128 | x.x.x.129 | x.x.x.254 | x.x.x.255 |

---

```
/26 = 8+8+8+2
255       255       255       192 = (128+64)
11111111  11111111  11111111  11000000

N = first value after last 255 e.g. 192
network increment:    256 - N = 256 - 192 = 64
number of subnets:    256 / 64 = 4
first valid address:  network id + 1
last valid address:   next network id - 2
```

| network id | first valid address | last valid address | broadcast address |
|:----------:|:-------------------:|:------------------:|:-----------------:|
| x.x.x.0 | x.x.x.1 | x.x.x.62 | x.x.x.63 |
| x.x.x.64 | x.x.x.65 | x.x.x.126 | x.x.x.127 |
| x.x.x.128 | x.x.x.129 | x.x.x.190 | x.x.x.191 |
| x.x.x.192 | x.x.x.193 | x.x.x.254 | x.x.x.255 |

---

```
/27 = 8+8+8+3
255       255       255       224 = (128+64+32)
11111111  11111111  11111111  11100000

N = first value after last 255 e.g. 224
network increment:    256 - N = 256 - 224 = 32
number of subnets:    256 / 32 = 8
first valid address:  network id + 1
last valid address:   next network id - 2
```

| network id | first valid address | last valid address | broadcast address |
|:----------:|:-------------------:|:------------------:|:-----------------:|
| x.x.x.0 | x.x.x.1 | x.x.x.30 | x.x.x.31 |
| x.x.x.32 | x.x.x.33 | x.x.x.62 | x.x.x.63 |
| x.x.x.64 | x.x.x.65 | x.x.x.94 | x.x.x.95 |
| x.x.x.96 | x.x.x.97 | x.x.x.126 | x.x.x.127 |
| x.x.x.128 | x.x.x.129 | x.x.x.158 | x.x.x.159 |
| x.x.x.160 | x.x.x.161 | x.x.x.190 | x.x.x.191 |
| x.x.x.192 | x.x.x.193 | x.x.x.222 | x.x.x.223 |
| x.x.x.224 | x.x.x.225 | x.x.x.254 | x.x.x.255 |