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Module - 3 Network Subnetting

2026-08-29 07:44

Tags: #Network

Author: Duke Hsu

Topic

  1. Problem with Large Broadcast Domains
  2. Resons for Subnetting
  3. Subnetting an IPv4 Network

1. Problem with Large Broadcast Domains

Slow network operations and device operations

  • Slow network

    • broadcast traffic

  • Slow device

    • must accept and process each broadcast packet

  • Network Slowdowns

  • Bandwidth Waste
  • Broadcast Storms
  • Security Vulnerabilities

Solution - Reduce the size of the network to create smaller broadcast domains in a process called subnetting. - VLANs - Routers and Layer 3 Switches

2. Resons for Subnetting

  • Quickly find the problem Location
  • Organizational Unit
  • Determined by device Type

3. Subnetting an IPv4 Network

  • Prefix length and the subnet mask are different ways of identifying the network portion of an address.
  • Subnets are created by borrowing host bits fro network bits.
  • More host bits borrowed , the more subnets that can be defined.

Subnetting Formula

  • Calculate number of Subnets formula : \(2^n\)
  • \(n\) is bits borrowed
  • Calculate number of Hosts formula: \(2^n - 2\)
  • \(n\) is the number of bits remaining int the host field

image.png

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3.1 Host bits borrowing example

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Example: /24 is the default CIDR

  • /25 - Borrowing 1 bit from the fourth octet creates 2 subnets supporting 126 hosts each .
  • /26 - Borrowing 2 bit from the fourth octet creates 4 subnets supporting 64 hosts each .
  • /27 - Borrowing 3 bit from the fourth octet creates 8 subnets supporting 32 hosts each .
  • /28 - Borrowing 4 bit from the fourth octet creates 16 subnets supporting 16 hosts each .
  • /29 - Borrowing 5 bit from the fourth octet creates 32 subnets supporting 8 hosts each .
  • /30 - Borrowing 6 bit from the fourth octet creates 64 subnets supporting 4 hosts each .

Network ID dan Broadcast Address

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Example - pkg

subnetting-example-topology.png

3.2 Magic Number

  • Magic number technique used to calculate subnets
  • Magic number is simple the place value of the last one in the subnet mask

subnetting-magical-number.jpg

  • Magic number is 32
  • Magic number in octet 4, therefor subnetting start 192.168.0/27
  • 192.168.1.0/27 -192.168.1.224/27

References

https://sysadminetwork.wordpress.com/2017/05/30/tcpiptcpip-internet-layer/

https://sysadminetwork.wordpress.com/2017/06/05/soal-subnetting/