Important concepts related to subnets

Let us quickly go through some important concepts related to subnets:

  • A network may be divided into sub networks for easy maintenance and security concerns. These sub networks are referred to as subnets.

  • Additional bits from the HID will be used for subnetting. The netmask for a subnet will represent the combination of NID bits (without subnet identifier bits) and subnet identifier bits, and this combined bit mask is usually called a subnet mask. 

  • Consider a CIDR block format of Here we have 24 NID bits and 8 HID bits. If we use 2 bits for subnet identification, then the subnet mask for a subnet becomes /26. The HID will now have 6 bits. 

  • Bigger the size of the subnet, the subnet mask will be smaller and vice versa. This is because subnet size is dependent on the HID bits which is calculated by subtracting subnet mask (NID bits) from total bits. For example, for a subnet with a subnet mask of /24, there are 8 HID bits and hence has 256 (2 to the power of 8) IP addresses. For a subnet with a subnet mask of /26, there are 6 HID bits and hence has 64 (2 to the power of 6) IP addresses. 

  • First IP address of a subnet represent the subnet ID and the last IP address represent the subnet's directed broadcast address. Therefore, we cannot use the first and last IP address of a subnet for hosts. 


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