There are four octet in IPV4. Each octet has 8 bits. So, there are 32 bits in IPV4. IPV4 is based on binary (0, 1) and decimal (0 to 9).

``````

1st Octet                    2nd Octet                    3rdOctet                     4th Octet
8bits                         8bits                       8 bits                      8 bits
27 26 25 24 23 22 21 20       27 26 25 24 23 22 21 20       27 26 25 24 23 22 21 20         27 26 25 24 23 22 21 20
255.255.255.255               255.255.255.255             255.255.255.255             255.255.255.255

How to become 255.255.255.255 ? As each octet has 8 bit and is based on binary....
27 = 128,  26 = 64,  25 = 32, 24 = 16,  23 = 8,  22 = 4, 21 = 2,  20 = 1
128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255

Let's try to change from the Decimal to the Binary....
Decmial                       =                        Binary
1st Octet     2nd Octet    3rd Octet   4th Octet               1stOctet      2nd Octet    3rd Octet    4th Octet
192       .    168      .   1         .  0          =        11000000  . 10101000  . 00000001 .  00000000
223       .    200      .   100       .  25         =        11011111  .  11001000  . 01100100 . 00011001
172       .    16       .   31        .  87         =        10101100  .  00010000  . 00011111 . 01010111

Let's explore 192.168.1.0/24....
Usable IP Addresses = 192.168.1.1 to 192.168.1.254

Let's explore 192.168.1.0/25....
Usable IP Addresses = 192.168.1.1 to 192.168.1.126

Usable IP Addresses = 192.168.1.129 to 192.168.1.254

Let's explore 192.168.1.0/26...
Usable IP Addresses = 192.168.1.1 to 192.168.1.62

Usable IP Addresses = 192.168.1.65 to 192.168.1.126

Usable IP Addresses = 192.168.1.129 to 192.168.1.190

Usable IP Addresses = 192.168.1.193 to 192.168.1.254

(Note: The Subnet Mask is reverse proportional to host range. The larger the subnet mask, the smaller the host range.)

The following table is formula for Ipv4 addresses (Class C). I've made it when I was in Singapore Polytechnic.

Binary      Decimal     Prefix     Subnet Mask      Count of Networks      Usable Ip Range
-              -         /24      255.255.255.0             1                    254
27           128          /25       255.255.255.128          2                    126
26           64           /26       255.255.255.192          4                     62
25           32           /27       255.255.255.224          8                     30
24           16           /28       255.255.255.240         16                    14
23            8           /29       255.255.255.248          32                     6
22            4            30       255.255.255.252          64                     2
``````