Subnetting: Step-by-Step Notes (Beginner Level)

0) What problem does subnetting solve?

• We split one IP network into smaller networks (subnets) to:
  • organize devices (by room/team/VLAN),
  • reduce broadcast traffic,
  • improve security/control,
  • match the number of hosts we actually need.

1) IPv4 address anatomy

• IPv4 address = 32 bits (four numbers 0–255), e.g. 192.168.1.34
• Two parts:
  • Network bits (which subnet you’re on)
  • Host bits (which device on that subnet)
• The subnet mask (or CIDR prefix) tells where we “cut” between network/host.
  • Examples:
    • /24 ↔ mask 255.255.255.0
    • /26 ↔ mask 255.255.255.192

2) Private IP ranges (safe for LANs)

• 10.0.0.0 – 10.255.255.255

• 172.16.0.0 – 172.31.255.255

• 192.168.0.0 – 192.168.255.255

  (Use these for your sandbox network.)

3) Subnet mask & CIDR quick links

Common /prefix ↔ dotted mask (last octet shown):

• /24 → 255.255.255.0 (0 host bits in last octet? No → 8 host bits)
• /25 → 255.255.255.128
• /26 → 255.255.255.192
• /27 → 255.255.255.224
• /28 → 255.255.255.240
• /29 → 255.255.255.248
• /30 → 255.255.255.252