Subnetting and the Magic Number Method

Overview

• Manual subnetting process
  • Writing down CIDR notation
  • Determining decimal mask
  • Binary version of subnet mask
  • Calculate number of networks and hosts per network

Given Task

• Network with IP address 192.16 needing subnetting
• Requirement: More than one network, 40 devices per subnet
• Manually create a chart to determine hosts per subnet
• Slash 27 supports 30 devices per subnet, Slash 26 supports 62 hosts per network on four subnets

Manual Calculations

• Write binary version of IP address and subnet mask
• Determine network part, subnet bits, and host bits
• Example: 24 network bits, 2 subnet bits, 6 host bits
• Total subnets: 2^2 = 4
• Hosts per subnet: 2^6 - 2 = 62

Key Addresses to Determine

• Network address (subnet ID)
• Broadcast address
• First available host address
• Last available host address

Challenges

• Time-consuming calculations
• Need shortcut for exam efficiency

Magic Number Method

• Shortcut for subnetting
• Perform most calculations mentally or with minimal math
• Useful charts can assist

Creating Useful Charts

• Write down CIDR block notations and corresponding decimal subnet mask
• Identify repeating values (e.g., 255.128.0.0 for /9 and /17)
• Use shortcuts from these patterns
• Creating charts showing interesting octets and magic numbers (128, 64, 32, etc.)
• Separation of host ranges for easy reference

Steps in Magic Number Method

1. Convert subnet mask to decimal
2. Identify the interesting octet
3. Calculate magic number (256 - interesting octet value)
4. Calculate host range, network address, and broadcast address

Example Calculation

1. Identify Subnet ID
   • IP address: 165.245.77.14
   • Subnet mask: 255.255.240.0
   • Interesting octet: 240
   • Magic number: 256 - 240 = 16
2. Calculate Broadcast Address
   • Subnet ID: 165.245.64.0
   • Add magic number, subtract 1 for broadcast: 64 + 16 - 1 = 79
   • Broadcast address: 165.245.79.255
3. Determine Host Range
   • First host: Subnet ID + 1 = 165.245.64.1
   • Last host: Broadcast address - 1 = 165.245.79.254

Another Example Using Shortcuts

• IP address: 10.180.122.244
• Subnet mask: 255.248.0.0
• Interesting octet: Second octet, value 248
• Magic number: 256 - 248 = 8
• Subnet ID: 10.176.0.0
• Broadcast address: 10.183.255.255
• Host range: First host = 10.176.0.1, Last host = 10.183.255.254

Final Summary

• Example IP: 172.16.242.133
• CIDR /27
• Subnet mask: 255.255.255.224
• Magic number: 32
• Subnet ID: 172.16.242.128
• Broadcast address: 172.16.242.159
• Host range: First host = 172.16.242.129, Last host = 172.16.242.158

Practice Recommendation

• Practice with different IP addresses and CIDR notations
• Identify shortcuts
• Perform subnetting tasks quickly for exams