IPv4 Address Classes & Subnet Masks Explained

Every computer on a network needs an IP address to communicate. IPv4 addresses are divided into classes to help organize networks based on size and purpose.
Along with this, subnet masks determine which part of the IP represents the network and which part represents the host.

This guide breaks it all down simply for beginners and IT enthusiasts.

What is a Subnet Mask? ​

A subnet mask is a 32-bit number that separates the network ID from the host ID in an IP address.

• Network Portion: Identifies the network your device is part of.
• Host Portion: Identifies the specific device within that network.

Visual Example:

• Black Part: Network
• Yellow Part: Host

Knowing subnet masks is crucial for IP planning, network design, and efficient data routing.

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IPv4 Address Classes ​

IPv4 addresses are divided into five classes (A, B, C, D, E), but for most IT and networking purposes, we focus on A, B, and C.

Class A: Huge Networks ​

• Subnet Mask: 255.0.0.0
• Network ID: 8 bits
• Host ID: 24 bits
• Number of Hosts: ~16 million per network
• Best Use: Very large organizations, ISPs, and global networks

Example: A multinational company with offices in hundreds of cities could use Class A IPs for all devices.

Class B: Medium Networks ​

• Subnet Mask: 255.255.0.0
• Network ID: 16 bits
• Host ID: 16 bits
• Number of Hosts: ~65,000 per network
• Best Use: Medium-sized organizations, universities, and regional networks

Example: A university campus network connecting thousands of students and faculty would use Class B IPs.

Class C: Small Networks ​

• Subnet Mask: 255.255.255.0
• Network ID: 24 bits
• Host ID: 8 bits
• Number of Hosts: Up to 254 per network
• Best Use: Small businesses, home networks, and offices

Example: A small office with 50 computers can easily use Class C IPs without running out of addresses.

Loopback: Test Your Own Computer ​

• IP Address: 127.0.0.1
• Subnet Mask: 255.255.255.255
• Purpose: Testing your own computer’s network stack

Example: Pinging 127.0.0.1 checks if your network interface and TCP/IP are working correctly.

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Quick Class Comparison Table ​

Class	Subnet Mask	Network Bits	Host Bits	Max Hosts	Use Case
Class A	255.0.0.0	8	24	16M+	Huge networks
Class B	255.255.0.0	16	16	65K	Medium networks
Class C	255.255.255.0	24	8	254	Small networks
Loopback	255.255.255.255	32	0	1	Self-testing

Why Understanding IP Classes & Subnets is Important ​

1. Efficient Network Design: Helps IT admins plan IP allocation properly.
2. Troubleshooting: Quickly identify network vs host issues.
3. Security: Proper subnetting can isolate segments and reduce attack surfaces.
4. Routing: Routers rely on network IDs to forward packets correctly.

Tip: Using Class C for small networks keeps things simple, while Class A/B is suitable for enterprise setups.

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Practical Examples ​

Example 1: Small Office Network

• Office has 100 computers.
• Use Class C: 192.168.1.0/24
• Network = 192.168.1 → Host = 1 to 254

Example 2: University Campus Network

• Thousands of devices across multiple buildings.
• Use Class B: 172.16.0.0/16
• Network = 172.16 → Host = 0.1 to 255.255

Example 3: ISP or Global Network

• Millions of devices globally.
• Use Class A: 10.0.0.0/8
• Network = 10 → Host = 0.0.1 to 255.255.255

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Key Takeaways ​

• Class A: Big networks (millions of hosts)
• Class B: Medium networks (~65k hosts)
• Class C: Small networks (<254 hosts)
• Loopback: Test your own machine
• Subnet masks split IPs into network + host, critical for network planning

Mastering IP classes and subnetting is a core skill for networking, IT, and cybersecurity professionals.