Subnetting: Magic Number Method

Jun 4, 2024

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Subnetting IP Addresses: Magic Number Method

Introduction

  • Subnetting is the process of dividing an IP address into multiple networks.
  • The goal is to support more than one network address and a specific number of devices per subnet.

Manual Subnetting Process Recap

  1. Determine CIDR notation and associated decimal mask.
  2. Convert the subnet mask to binary.
  3. Calculate the number of networks and hosts per network.
  4. Identify essential addresses:
    • Network address (Subnet ID)
    • Broadcast address
    • First available host address
    • Last available host address
  • Note: This process involves a lot of time-consuming calculations.

Shortcut: Magic Number Method

  • Simplifies the subnetting process.
  • Allows subnet calculations to be done quickly with minimal mathematical operations.

Steps in the Magic Number Method

  1. Convert Subnet Mask to Decimal: Identify CIDR block notation and determine its decimal form.
  2. Identify the Interesting Octet: The octet in the subnet mask that is neither 255 nor 0.
  3. Calculate Magic Number: 256 minus the value of the interesting octet.
  4. Calculate Host Range:
    • Subnet ID:
      • Use the IP address's copy action for any octet associated with a 255 in the subnet mask.
      • For octets with a 0 value in the subnet mask, copy down the zero.
      • The interesting octet value is the multiple of the magic number within which the IP address falls.
    • Broadcast Address:
      • Add the magic number to the subnet ID and subtract one.
      • Use the copy action for 255s and the value 255 for subnet mask's zero octets.
  5. Determine Host Range:
    • First Host Address: Subnet ID + 1.
    • Last Host Address: Broadcast Address - 1.

Calculation Examples

  1. Example 1: IP Address 165.245.77.14 with Subnet Mask 255.255.240.0

    • Interesting Octet: 240.
    • Magic Number: 256 - 240 = 16.
    • Subnet ID blocks: multiples of 16, 77 falls within 64-80.
    • Subnet ID: 165.245.64.0.
    • Broadcast Address: 165.245.79.255.
    • First Host: 165.245.64.1.
    • Last Host: 165.245.79.254.

  2. Example 2: IP Address 10.180.122.244 with Subnet Mask 255.248.0.0

    • Interesting Octet: 248.
    • Magic Number: 256 - 248 = 8.
    • Subnet ID blocks: multiples of 8, 180 falls within 176-183.
    • Subnet ID: 10.176.0.0.
    • Broadcast Address: 10.183.255.255.
    • First Host: 10.176.0.1.
    • Last Host: 10.183.255.254.

Utilizing Charts for Quick Calculations

  • Create or refer to charts that list CIDR notations with their decimal equivalents and interesting octet values.
  • Charts can simplify finding magic numbers and host ranges for various subnet sizes.

Summary

  1. Start by identifying CIDR notation and subnet masks.
  2. Use the magic number to simplify the calculations.
  3. Determine all necessary subnetting addresses.
  4. Practice multiple times to identify personal shortcuts.

This method reduces the time needed to perform subnetting, making it feasible for exam scenarios.