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Learning / networking

Subnetting Basics

beginner ipv4subnettingaddressing

Subnetting is how a large IP network gets divided into smaller, manageable pieces. Every network engineer needs this instinctively — not just for exams, but for reading a topology and immediately knowing how many usable hosts a subnet has.

Why it matters

A /24 network gives you 254 usable hosts. A /28 gives you 14. Getting this wrong in the field means either wasting address space or running out of room for new devices.

The quick method

  1. Convert the subnet mask to binary.
  2. Count the borrowed bits.
  3. 2^(borrowed bits) = number of subnets
  4. 2^(host bits) - 2 = usable hosts per subnet
/26 = 255.255.255.192
Borrowed bits: 2 → 4 subnets
Host bits: 6 → 62 usable hosts per subnet

Key takeaway

Practice this until it’s instant — in real troubleshooting you rarely have time to sit down and do long division on a whiteboard.