7 Second Subnetting

Feb 19, 2025

Lecture Notes: 7c Subnetting Shortcut

Overview

  • Previous Topics: Manual conversion of IP addresses to binary and back, Magic Number Method
  • 7c Subnetting: A shortcut for subnetting without complex math, using a predefined chart
    • No need for binary-to-decimal conversion
    • Only simple addition/subtraction for IP calculations

Key Concepts

Subnetting Process

  • Objective: Split a network into smaller subnets
  • Examples:
    • Network with 256 hosts -> Various smaller subnets (e.g., 128, 192, 224)

7c Subnetting Chart

  • Components:
    • Subnet masks (for quick conversion between CIDR and decimal)
    • Networks column (start with 2, double to 256)
    • Addresses column (start with 128, halve to 1)
    • Subnet mask in decimal column (for conversion)

Calculation Process

  • Four-Step Process:
    1. Convert IP and subnet mask to decimal
    2. Determine subnet address
    3. Identify broadcast address
    4. Calculate first and last usable IP addresses

Step-by-Step Examples

Example 1

  • IP/Subnet: 165.245.12.188/24
    • Convert /24 to 255.255.255.0
    • Network Address: 165.245.12.0
    • Broadcast Address: 165.245.12.255
    • First IP: 165.245.12.1
    • Last IP: 165.245.12.254

Example 2

  • IP/Subnet: 165.245.12.188/26
    • Convert /26 to 255.255.255.192
    • Network Address: 165.245.12.64
    • Broadcast Address: 165.245.12.127
    • First IP: 165.245.12.65
    • Last IP: 165.245.12.126

Additional Examples

  • Example 3:

    • IP/Subnet: 165.245.12.188/20
    • Convert /20 to 255.255.240.0
    • Network Address: 165.245.0.0
    • Broadcast Address: 165.245.15.255
    • First IP: 165.245.0.1
    • Last IP: 165.245.15.254

  • Example 4:

    • IP/Subnet: 18.172.2.77/11
    • Convert /11 to 255.224.0.0
    • Network Address: 18.160.0.0
    • Broadcast Address: 18.191.255.255
    • First IP: 18.160.0.1
    • Last IP: 18.191.255.254

  • Example 5:

    • IP/Subnet: 18.172.2.77/17
    • Convert /17 to 255.255.128.0
    • Network Address: 18.172.128.0
    • Broadcast Address: 18.172.255.255
    • First IP: 18.172.128.1
    • Last IP: 18.172.255.254

Tips for Exam Preparation

  • Practice creating charts quickly
  • Consider using fine-tip dry erase pens at testing centers
  • Evaluate different shortcuts (7c, Magic Number, etc.) and choose the best fit

Conclusion

  • The 7c subnetting process simplifies subnet calculations
  • Practice with the chart can improve speed and accuracy
  • Choose a method that complements your exam strategy

Full transcript