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📘 08-04: Classful Networks

Network Systems

Module 08: Subnets and VLANs

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📌 What are Classful Networks?

Classful networking is an older method of IP address allocation where IP addresses are divided into fixed classes.

Each class defines: - a default subnet mask - a fixed division between network and host portions

💡 Why Classful Networks Were Used

  • Simple and easy to understand
  • Standardized network sizes
  • No need to specify subnet masks explicitly

🔢 IPv4 Address Classes

IPv4 addresses are divided into five classes:

🔹 Class A

  • Range: 0.0.0.0 → 127.255.255.255
  • Default Subnet Mask:
  • 255.0.0.0
  • 11111111.00000000.00000000.00000000
  • /8
  • Network portion: first 8 bits
  • Host portion: last 24 bits

Note: - 0.0.0.0 → reserved (unspecified address, default route: 0.0.0.0/0)
- 127.0.0.0 → 127.255.255.255 → loopback range
- Usable range: 1.0.0.0 → 126.255.255.255

🔹 Class B

  • Range: 128.0.0.0 → 191.255.255.255
  • Default Subnet Mask:
  • 255.255.0.0
  • 11111111.11111111.00000000.00000000
  • /16
  • Network portion: first 16 bits
  • Host portion: last 16 bits

🔹 Class C

  • Range: 192.0.0.0 → 223.255.255.255
  • Default Subnet Mask:
  • 255.255.255.0
  • 11111111.11111111.11111111.00000000
  • /24
  • Network portion: first 24 bits
  • Host portion: last 8 bits

🔹 Class D

  • Range: 224.0.0.0 → 239.255.255.255
  • Purpose: Multicast communication
  • No subnet mask, network portion, or host portion defined

🔹 Class E

  • Range: 240.0.0.0 → 255.255.255.255
  • Purpose: Experimental use
  • No subnet mask, network portion, or host portion defined

📌 Key Observation

The first three classes (A, B, C) have fixed subnet masks.

This means: - network and host portions are predefined
- there is no flexibility in dividing the network

📌 Limitations of Classful Networking

  • Inefficient use of IP addresses
  • Fixed network sizes (too large or too small)
  • Lack of flexibility

📌 Transition to Modern Networking

Due to these limitations, classful networking was replaced by:

  • Classless Inter-Domain Routing (CIDR)
  • Subnetting

💡 Key Idea

Classful networking uses fixed network boundaries,

while modern networking allows flexible boundaries using subnet masks.