Subnetting with Magic Number Method

Overview

This lecture covers the easiest method to subnet IP networks, focusing on class C addresses using the "magic number" approach to quickly determine network, broadcast, host counts, and ranges for exams or networking tasks.

Subnetting Basics

• Subnetting divides an IP network into smaller sub-networks for efficient management and security.
• Key questions asked: find network address, broadcast address, total hosts, usable hosts, and subnet IP range.
• You are usually given an IP address and a subnet mask (slash or dotted notation).

The Magic Number Method (Class C)

• For a class C address, use 32 (total bits) minus the subnet mask number (slash notation) to get the number of host bits.
• Example: For /28, 32 - 28 = 4.
• Calculate the magic number: 2 to the power of host bits (e.g., 2⁴ = 16).
• The magic number is used to find network ranges and addresses.

Creating the T-Chart for Subnetting

• List network start values in one column and broadcast values in another.
• Start with 0, add the magic number for each subsequent row (e.g., 0, 16, 32, 48…).
• Broadcast address is always one less than the next network address (e.g., 16-1=15, 32-1=31, etc.).
• Identify where the given IP's last octet falls to determine subnet range.

Determining Key Subnet Values

• Network address: network start value in the range.
• Broadcast address: end value in the range.
• Total hosts: magic number.
• Usable hosts: magic number minus 2 (for network and broadcast).
• Usable range: network+1 to broadcast-1.

Using Dotted Decimal Notation

• For subnet mask in dotted decimal (e.g., 255.255.255.224), subtract last octet from 256 for magic number (256-224=32).
• Proceed with T-chart steps using this magic number.

Example Application

• Given 192.168.10.40/28, magic number is 16: networks are 0,16,32,48; broadcast for 32 network is 47.
• Given IP 10.50.20.100 with 255.255.255.224, magic number is 32: networks are 0,32,64,96, etc.; broadcast for 96 network is 127.

Key Terms & Definitions

• Subnetting — Dividing a network into smaller, manageable segments.
• Network Address — First address in the subnet, not assigned to hosts.
• Broadcast Address — Last address in the subnet; used for communication to all hosts.
• Usable Hosts — Addresses assignable to devices (total hosts minus network and broadcast).
• Slash Notation (/28) — Short form for subnet mask indicating number of network bits.
• Dotted Decimal Notation — Subnet mask written as four octets (e.g., 255.255.255.224).
• Magic Number — The size of each subnet’s address block, calculated for subnetting.

Action Items / Next Steps

• Practice subnetting problems using both slash and dotted decimal notation.
• Create T-charts for different IP addresses and subnet masks.
• Prepare to apply this method to class B addresses in future lessons.