Understanding Subnetting and CIDR

1. Introduction to Subnetting

Subnetting is the process of dividing a larger network into smaller sub-networks (subnets). This helps in efficient IP address allocation, improved network security, and better performance.

A subnet mask is used to define the boundary of each subnet by determining which part of an IP address belongs to the network and which part belongs to the host.

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2. Understanding CIDR (Classless Inter-Domain Routing)

CIDR notation represents a subnet mask by using a slash (/) followed by the number of bits used for the network portion.

For example:

• 192.168.1.0/24 means that the first 24 bits are for the network, and the remaining 8 bits are for hosts.

• The subnet mask for /24 is 255.255.255.0, meaning 256 total IPs (254 usable hosts).

The formula to calculate the total IPs:

Total IPs = 2^(32 - CIDR Prefix)
Usable Hosts = Total IPs - 2 (Network & Broadcast Address)

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3. How to Determine the Number of Subnets and Hosts

Step-by-Step Process

1. Identify the given IP address and CIDR prefix.

2. Determine the subnet mask.

3. Calculate total subnets using borrowed bits.

4. Calculate the number of usable hosts per subnet.

5. Identify the subnet IP ranges.

Subnet Mask Creation

A subnet mask is a 32-bit number where:

• Network bits = 1s

• Host bits = 0s

For example:

• /24 → 255.255.255.0 (First 24 bits as 1s, last 8 bits as 0s)

• /26 → 255.255.255.192 (First 26 bits as 1s, last 6 bits as 0s)

• /30 → 255.255.255.252 (First 30 bits as 1s, last 2 bits as 0s)

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4. Examples of Subnetting

Example 1: /24 Subnet

Given:

• IP Address: 192.168.1.0/24

• Subnet Mask: 255.255.255.0

• Total IPs: 2^(32-24) = 256

• Usable Hosts: 256 - 2 = 254

Subnet Breakdown:

This /24 network is one big subnet with 254 usable IPs.

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Example 2: /26 Subnet (Dividing a /24 into 4 subnets)

Given:

• IP Address: 10.0.0.0/26

• Subnet Mask: 255.255.255.192

• Total IPs: 2^(32-26) = 64

• Usable Hosts: 64 - 2 = 62

• Number of Subnets from /24: 2^(26-24) = 4

Subnet Breakdown:

Each subnet has 62 usable IPs.

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Example 3: /30 Subnet (For Point-to-Point Connections)

Given:

• IP Address: 172.16.0.0/30

• Subnet Mask: 255.255.255.252

• Total IPs: 2^(32-30) = 4

• Usable Hosts: 4 - 2 = 2

Subnet Breakdown:

Each subnet only has 2 usable IPs, making it ideal for point-to-point connections.

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5. Importance of Subnet Mask

A subnet mask is critical for:

• Defining Network Boundaries: Separates networks and hosts.

• Routing Efficiency: Ensures that data reaches the correct subnet.

• Security: Limits communication between devices by segmenting the network.

• Optimized IP Utilization: Prevents wastage of IP addresses.

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6. Summary

This guide provides a step-by-step approach to subnetting, making it easier to determine the number of subnets and hosts.