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📘 08-06: Subnetting Basics

Network Systems

Module 08: Subnets and VLANs

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❓ What is Subnetting?

Subnetting is the process of dividing a network into smaller sub-networks (subnets).

It allows a large network to be split into multiple smaller and manageable networks.

💡 Why Subnetting is Needed

  • Improves network performance
  • Enhances security by isolating segments
  • Efficient use of IP addresses
  • Better network management

📌 Relationship with Classful Networks

In classful networking: - network and host portions are fixed
- network portion always ends at the end of an octet

Subnetting allows: - modification of the default boundary
- creation of multiple smaller networks from a single network
- network portion can extend into the next octet (bit-level division)

🧮 How Subnetting Works

Subnetting works by borrowing bits from the host portion.

  • Some host bits are converted into network bits
  • This creates multiple smaller subnets

📌 Example

Original Network: - IP Address: 192.168.1.0
- Default Subnet Mask: 255.255.255.0 (/24)

After Subnetting (/26): - New Subnet Mask: 255.255.255.192 (/26)

Explanation: - Original network bits: 24
- New network bits: 26
- Borrowed bits: 2

🧮 Number of Subnets

Formula: Number of subnets = 2ⁿ

Where: - n = number of borrowed bits

Example: 2² = 4 subnets

🧮 Number of Addresses and Hosts per Subnet

Formula: - Total addresses per subnet = 2ʰ
- Usable hosts per subnet = 2ʰ − 2

Where: - h = number of remaining host bits

Important: - First address → Network ID (reserved)
- Last address → Broadcast ID (reserved)

🔹 Example

6 host bits → 2⁶ = 64 total addresses

  • Network Address → first address
  • Broadcast Address → last address

Usable hosts = 64 − 2 = 62

📌 Block Size

Block size determines the increment between subnet ranges.

Formula: Block Size = 256 − (subnet mask value in the changing octet)

Example: Subnet Mask: 255.255.255.192

Block Size = 256 − 192 = 64

Subnet ranges increase as: 0, 64, 128, 192

💡 Why Block Size Matters

  • Helps identify subnet ranges
  • Determines network and broadcast addresses
  • Simplifies subnet calculations

🎯 Choosing the Right Subnet Size

For design questions, start with the host requirement, not with the mask.

Need Smallest common prefix Usable hosts
2 hosts /30 2
14 hosts /28 14
30 hosts /27 30
62 hosts /26 62
126 hosts /25 126
254 hosts /24 254
510 hosts /23 510

Process:

  1. Find the smallest host-bit value where 2^h - 2 meets the requirement.
  2. Convert host bits to prefix length: 32 - h.
  3. Check the block size to identify the network and broadcast range.

Example: a department needs 58 hosts. 2^5 - 2 = 30 is too small, but 2^6 - 2 = 62, so the smallest subnet is /26.

💡 Key Idea

Classful networking uses fixed boundaries.

👉 Subnetting introduces flexibility by allowing custom network boundaries.