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Subnetting

Subnetting is the process of dividing a large network into smaller, more manageable networks called subnets.
It is done by manipulating the subnet mask to allocate more bits to the network portion of an IP address and fewer bits to the host portion.

Table of Contents

  1. What is Subnetting?
  2. Why Do We Need Subnetting?
  3. How Subnetting Works
  4. Subnetting Example
  5. Subnetting in Binary
  6. Advantages of Subnetting
  7. Subnetting, CIDR, and VLSM
  8. Further Reading

What is Subnetting?

  • In IPv4, each address has two parts:
    • Network ID – identifies the overall network.
    • Host ID – identifies the specific device (host) in that network.

Subnetting allows us to break a single network into smaller sub-networks by borrowing bits from the host portion and using them as additional network bits.

Why Do We Need Subnetting?

  1. Efficient IP Address Usage

    • Without subnetting, large blocks of addresses are wasted.
    • Subnetting ensures we use IP space efficiently.

  2. Reduced Network Congestion

    • Smaller subnets mean fewer devices per broadcast domain.
    • This reduces unnecessary traffic.

  3. Improved Security

    • Subnets can be used to isolate different departments or services.

  4. Simplified Management

    • Easier to troubleshoot and manage smaller networks.

How Subnetting Works

Step 1: Determine the Network Address

Start with the IP address and default subnet mask.
Example:

  • IP: 192.168.1.0
  • Default mask: 255.255.255.0 (/24)

This gives 1 network with 254 hosts.

Step 2: Choose a Subnet Mask

Decide how many subnets you need or how many hosts per subnet.

  • Borrow bits from the host portion.
  • Each borrowed bit doubles the number of subnets.
  • Each borrowed bit halves the number of hosts per subnet.

Step 3: Calculate Number of Subnets and Hosts

Formulas:

  • Number of subnets = 2^n (where n = number of borrowed bits)
  • Number of hosts per subnet = (2^h) - 2 (where h = number of host bits left)
    We subtract 2 for the network and broadcast addresses.

Step 4: Find Subnet Ranges

Each subnet has:

  • A network address (first address).
  • A host range (usable addresses).
  • A broadcast address (last address).

Subnetting Example

Suppose we have:

  • Network: 192.168.1.0/24
  • We want 4 subnets.
  1. /24 means 8 host bits.
  2. To create 4 subnets, we borrow 2 bits:
    • New prefix = /26
    • New mask = 255.255.255.192

Calculation:

  • Number of subnets = 2^2 = 4
  • Hosts per subnet = (2^6) - 2 = 62

Subnets:

Subnet # Network Address Usable Hosts Range Broadcast Address
1 192.168.1.0 192.168.1.1 – 192.168.1.62 192.168.1.63
2 192.168.1.64 192.168.1.65 – 192.168.1.126 192.168.1.127
3 192.168.1.128 192.168.1.129 – 192.168.1.190 192.168.1.191
4 192.168.1.192 192.168.1.193 – 192.168.1.254 192.168.1.255

Subnetting in Binary

Subnetting is essentially binary math.

Example:

  • IP: 192.168.1.0
  • Default mask: 11111111.11111111.11111111.00000000 (/24)

If we borrow 2 bits:

  • New mask: 11111111.11111111.11111111.11000000 (/26)

This creates 4 subnets. Each subnet’s increment is 64 in the last octet.

Advantages of Subnetting

  • Saves IP addresses by dividing large blocks.
  • Improves network performance (smaller broadcast domains).
  • Enhances security by isolating networks.
  • Enables better control and management of traffic.

Subnetting, CIDR, and VLSM

  • Subnetting is the foundation: dividing a network into smaller parts.
  • CIDR (Classless Inter-Domain Routing) introduced flexible prefix notation (e.g., /26) instead of rigid classes.
  • VLSM (Variable Length Subnet Masking) allows using different masks for different subnets within the same network, making address allocation even more efficient.

Further Reading