Those who start towards the path of CCENT realize IPv4 subnetting is a core component that continues throughout the rest of the Cisco certification tracks: CCNA, CCNP, and CCIE.
The IPv4 address is an address used to uniquely identify a device on an IP network. The address is made up of 32 binary bits, which can be divisible into a network portion and a host portion with the help of a subnet mask. The practice of dividing a network into two or more networks is called subnetting. Note that an IPv4 address has 32 binary bits that are broken into four octets (1 octet = 8 bits). Each octet is converted to decimal, separated by a period (dot). For this reason, an IP address is said to be expressed in dotted decimal format (for example, 192.168.1.0). The value in each octet ranges from 0 to 255 decimal, or 00000000 - 11111111 binary.
The Cisco Learning Network has developed an IPv4 subnet chart to help aid in your learning. This chart organizes Class A, B, and C networks and various subnets. The chart displays slash (/) notation (a compact representation of the associated routing prefix), netmask (32-bit mask used to divide an IP address into subnets and specify the network's available hosts), block size (the entire IP space that a network takes up), and the number of hosts to networks per subnet.