Subnetting

Dividing a larger IP network into smaller, more manageable segments is called subnetting / think of it like cutting a big cake into precise slices so each department or group gets exactly what they need.

Why Subnet?

• Organization & Management: Separate departments (e.g. Accounting, HR, IT) each get their own address block.
• Performance: Smaller broadcast domains reduce unnecessary traffic and collisions.
• Security: Isolate sensitive systems / guest Wi‑Fi can’t talk directly to your servers.
• Scalability: Plan growth by reserving address blocks for future use.
• Efficient IP Use: Fit the right‑sized network to each group, avoiding wasted addresses.

Key Concepts

• IP Address Structure:
  • 32 bits (IPv4) split into four 8‑bit “octets.”
  • Written in dotted‑decimal (e.g. 192.168.10.0).
• Subnet Mask:
  • Also 32 bits (dotted‑decimal or CIDR / notation).
  • Masks off the network portion from the host portion.
  • Example:
    • /24 mask = 255.255.255.0
    • /26 mask = 255.255.255.192
• CIDR Notation:
  • A shorthand that shows how many bits are “1” in the mask (e.g. 192.168.1.0/26).

How to Subnet: Step by Step

1. Determine Requirements
   • Number of subnets needed
   • Hosts per subnet (include room for growth + network/broadcast addresses).
2. Choose a Subnet Mask
   • Borrow bits from the host portion to create enough subnets.
   • Formula:
     • Subnets = 2ⁿ (n = bits borrowed)
     • Hosts per subnet = 2^(host bits) – 2
3. Calculate Subnet Increments
   • The increment = 256 – (last octet of mask).
   • Example /26: 256 – 192 = 64, so subnets start at .0, .64, .128, .192.
4. List Subnet Ranges
   • For 10.0.0.0/26:
     • Subnet 1: 10.0.0.0 – 10.0.0.63 (hosts .1–.62, broadcast .63)
     • Subnet 2: 10.0.0.64 – 10.0.0.127 (hosts .65–.126, broadcast .127)
     • …and so on.