Subnetting

Dividing Networks into Smaller Pieces

Subnetting is the practice of dividing a large network into smaller, more manageable sub-networks (subnets). It's like dividing a large office building into separate floors and departments — each section has its own address range and can be managed independently.

Subnetting is essential for efficient IP address usage, improved security, and better network performance.

Why Subnet?

• Efficiency — Without subnetting, a Class C network with 254 hosts might be wasted on a department of 10 people. Subnetting lets you allocate just the right number of addresses.
• Performance — Smaller subnets mean fewer devices on each broadcast domain. Broadcast traffic (like ARP requests) stays within the subnet instead of flooding the entire network.
• Security — Subnets can be separated by routers or firewalls, allowing you to control traffic between departments.
• Organization — Subnets mirror the organizational structure — different departments, locations, or functions get their own subnets.

How Subnetting Works

Subnetting borrows bits from the host portion of an IP address to create a subnet ID. The subnet mask determines how many bits are borrowed.

Example: Take the network 192.168.1.0/24 (254 usable hosts). If we borrow 2 bits for subnetting:

  Original:     192.168.1.0/24
  Subnetted:    192.168.1.0/26

  /26 means 26 bits for network + subnet, 6 bits for hosts

  Subnet mask:  255.255.255.192  (11111111.11111111.11111111.11000000)

  Subnets created:
  ┌─────────────────────┬─────────────────────────┬─────────────┐
  │ Subnet              │ Usable Host Range       │ Broadcast   │
  ├─────────────────────┼─────────────────────────┼─────────────┤
  │ 192.168.1.0/26      │ 192.168.1.1 - .62      │ 192.168.1.63│
  │ 192.168.1.64/26     │ 192.168.1.65 - .126    │ .127        │
  │ 192.168.1.128/26    │ 192.168.1.129 - .190   │ .191        │
  │ 192.168.1.192/26    │ 192.168.1.193 - .254   │ .255        │
  └─────────────────────┴─────────────────────────┴─────────────┘

We went from 1 network with 254 hosts to 4 subnets with 62 hosts each.

CIDR Notation

CIDR (Classless Inter-Domain Routing) uses a slash followed by the number of network bits:

  192.168.1.0/24    →  24 network bits, 8 host bits   → 254 hosts
  192.168.1.0/25    →  25 network bits, 7 host bits   → 126 hosts
  192.168.1.0/26    →  26 network bits, 6 host bits   → 62 hosts
  192.168.1.0/27    →  27 network bits, 5 host bits   → 30 hosts
  192.168.1.0/28    →  28 network bits, 4 host bits   → 14 hosts
  192.168.1.0/30    →  30 network bits, 2 host bits   → 2 hosts

The formula for usable hosts: 2^(host bits) - 2. We subtract 2 because the all-zeros address is the network address and the all-ones address is the broadcast address.

Subnetting Example

You have the network 10.0.0.0/8 and need to create subnets for 5 departments with 2000 hosts each. How many bits do you need to borrow?

For 2000 hosts: 2^11 = 2048, so we need 11 host bits. That means we borrow 32 - 11 = 21 bits from the host portion. But wait — we only need 5 subnets, so 3 bits (2^3 = 8 subnets) would suffice.

We borrow 3 bits: 10.0.0.0/11

  Subnet 1: 10.0.0.0/11     → 10.0.0.1 - 10.31.255.254
  Subnet 2: 10.32.0.0/11    → 10.32.0.1 - 10.63.255.254
  Subnet 3: 10.64.0.0/11    → 10.64.0.1 - 10.95.255.254
  Subnet 4: 10.96.0.0/11    → 10.96.0.1 - 10.127.255.254
  Subnet 5: 10.128.0.0/11   → 10.128.0.1 - 10.159.255.254

Each subnet has over 2 million addresses — more than enough for 2000 hosts.