16. How to Find the Number of Subnets Valid Hosts

Introduction

This tutorial provides a step-by-step guide on how to find the number of subnets and valid hosts in an IP addressing scheme, an essential skill for network engineers and those preparing for the CCNA exam. Understanding subnetting is crucial for efficient network management and optimization.

Step 1: Understand IP Address Structure

• IP Address Composition: An IP address is composed of two main parts: the network part and the host part.
• Classful Addressing: Familiarize yourself with the different classes of IP addresses (A, B, C) and their default subnet masks:
  • Class A: 1.0.0.0 to 126.0.0.0 (Default Mask: 255.0.0.0)
  • Class B: 128.0.0.0 to 191.255.0.0 (Default Mask: 255.255.0.0)
  • Class C: 192.0.0.0 to 223.255.255.0 (Default Mask: 255.255.255.0)

Step 2: Identify the Subnet Mask

• Custom Subnetting: If you are using a custom subnet mask, convert it into binary to determine how many bits are used for the network and how many for the hosts.
• Example: A subnet mask of 255.255.255.192 in binary would be represented as:

  11111111.11111111.11111111.11000000
  

• Count the Bits: Count the number of 1s for the network bits and 0s for the host bits.

Step 3: Calculate the Number of Subnets

• Formula: Use the formula ( 2^n ) where ( n ) is the number of bits borrowed from the host part for subnetting.
• Example: If you borrow 2 bits for subnetting from a Class C address:
  • Number of subnets = ( 2^2 = 4 )

Step 4: Calculate the Number of Valid Hosts

• Formula: Use the formula ( 2^h - 2 ) where ( h ) is the number of bits left for hosts (subtract 2 for the network and broadcast addresses).
• Example: If you have 6 bits left for hosts:
  • Number of valid hosts = ( 2^6 - 2 = 62 )

Step 5: Practical Application

• Subnetting Example: Given an IP address of 192.168.1.0 with a subnet mask of 255.255.255.192:
  • Subnets: Calculate using the method described in Step 3 to find 4 subnets.
  • Valid Hosts: Apply Step 4 to find 62 valid hosts per subnet.

Conclusion

In this tutorial, you learned how to find the number of subnets and valid hosts when subnetting an IP address. Remember to always consider the default classful addressing, identify the custom subnet mask, and apply the correct formulas for calculations. This foundational knowledge is vital for managing networks efficiently and preparing for certification exams like CCNA. Consider practicing with different IP addresses and subnet masks to strengthen your understanding.