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Quick Guide to Subnetting Techniques

Aug 8, 2024

Seven Second Subnetting Process

Introduction

  • Purpose: Shortcut for quick subnetting during certification exams.
  • Benefit: No complex math, just addition and subtraction.
  • Comparison: Similar to Magic Number Method.
  • Customization: Use elements from both methods as needed.

Requirements

  • Create Tables/Charts: Essential for quick reference.
  • Testing Environment: Options for whiteboard (physical or digital).
  • Preparation: Practice creating and using charts manually.

Process Overview

  • Subnet Mask Examples:
    • /24 (255.255.255.0): Single subnet.
    • /25 (255.255.255.128): Two subnets.
    • /27 (255.255.255.224): Eight subnets.
  • Chart Components:
    • CIDR block notations for each octet.
    • Network counts and addresses per subnet.
    • Decimal conversions for each mask.

Detailed Steps

  1. Convert IP and Subnet Mask to Decimal:
    • E.g., /24 -> 255.255.255.0
  2. Determine Network/Subnet Address:
    • Use the subnet mask to extract network portions.
  3. Calculate Broadcast Address:
    • Replace host portion with 255.
  4. First/Last Usable IPs:
    • First: Add 1 to network address.
    • Last: Subtract 1 from broadcast address.

Example Calculations

Example 1: IP 165.245.12.88 /24

  • Convert to Decimal:
    • IP: 165.245.12.88
    • Subnet Mask: 255.255.255.0
  • Network Address: 165.245.12.0
  • Broadcast Address: 165.245.12.255
  • Usable IPs: 165.245.12.1 to 165.245.12.254

Example 2: IP 165.245.12.88 /26

  • Convert to Decimal:
    • IP: 165.245.12.88
    • Subnet Mask: 255.255.255.192
  • Network Address: 165.245.12.64
  • Broadcast Address: 165.245.12.127
  • Usable IPs: 165.245.12.65 to 165.245.12.126

Example 3: IP 165.245.12.88 /20

  • Convert to Decimal:
    • IP: 165.245.12.88
    • Subnet Mask: 255.255.240.0
  • Network Address: 165.245.0.0
  • Broadcast Address: 165.245.15.255
  • Usable IPs: 165.245.0.1 to 165.245.15.254

Example 4: IP 18.172.200.77 /11

  • Convert to Decimal:
    • IP: 18.172.200.77
    • Subnet Mask: 255.224.0.0
  • Network Address: 18.160.0.0
  • Broadcast Address: 18.191.255.255
  • Usable IPs: 18.160.0.1 to 18.191.255.254

Example 5: IP 18.172.200.77 /17

  • Convert to Decimal:
    • IP: 18.172.200.77
    • Subnet Mask: 255.255.128.0
  • Network Address: 18.172.128.0
  • Broadcast Address: 18.172.255.255
  • Usable IPs: 18.172.128.1 to 18.172.255.254

Practical Tips

  • Preparation Time: Takes 1-2 minutes to create charts before the exam.
  • Tools: Bring your own fine-tip dry erase marker for better clarity.
  • Customization: Adapt methods to personal preference for optimal performance.

Full transcript