Subnetting means borrowing bits from the host portion to create several smaller networks out of a single one. It segments traffic, improves security and saves addresses.

The method

  • Work out how many subnets or hosts you need.
  • Borrow the right number of bits (2^bits subnets).
  • Calculate the new mask and the block size (increment).
  • List the ranges of each subnet.

Example: starting from 192.168.1.0/24, you borrow 2 bits to get four /26 subnets.

Starting network : 192.168.1.0 /24
New mask         : 255.255.255.192  (/26)
Block size       : 64
Subnet 1         : 192.168.1.0   /26
Subnet 2         : 192.168.1.64  /26
Subnet 3         : 192.168.1.128 /26
Subnet 4         : 192.168.1.192 /26

Each /26 gives 62 usable hosts (2^6 – 2). The block size (64 here) hands you the successive network addresses directly, which speeds up mental arithmetic without converting to binary every time.

Key point: borrowing bits creates subnets; the block size = 256 minus the value of the last octet of the mask. See Lab 11 – Subnetting a network.

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