Learn the complete subnet ranges for IPv4 Class A, B, and C networks. This guide explains default masks, CIDR notation, address ranges, and practical subnetting examples for networking and cybersecurity professionals.
IPv4 addressing remains a foundational concept in networking, security engineering, and system administration. Despite the shift toward IPv6, understanding IPv4 subnet ranges and classful addressing is essential for architects, administrators, and analysts working with enterprise networks, firewalls, cloud systems, and compliance-driven environments.
This guide provides a clear, structured overview of IPv4 class ranges, default subnet masks, CIDR blocks, and commonly used subnet ranges for Class A, Class B, and Class C networks.
Table of Contents
1. What Is Classful IPv4 Addressing?
Classful addressing was the original method used in IPv4 to segment network blocks into predefined ranges known as Class A, Class B, and Class C.
Although modern networks use CIDR (Classless Inter-Domain Routing), the classful model still matters because:
- Legacy systems and documentation reference classful boundaries
- Many certification exams (CCNA, CEH, Security+, OSCP) require understanding class ranges
- Public routable blocks historically follow classful allocation
- Class ranges provide a baseline for planning private networks
2. IPv4 Class Ranges and Default Subnet Masks
Below is a table summarizing the core attributes of IPv4 classes.
| Class | Starting Range | Ending Range | Default Subnet Mask | CIDR Notation | Number of Hosts per Network |
|---|---|---|---|---|---|
| A | 1.0.0.0 | 126.255.255.255 | 255.0.0.0 | /8 | 16,777,214 |
| B | 128.0.0.0 | 191.255.255.255 | 255.255.0.0 | /16 | 65,534 |
| C | 192.0.0.0 | 223.255.255.255 | 255.255.255.0 | /24 | 254 |
Special Notes:
- 127.0.0.0/8 is reserved for loopback, not Class A usage.
- 224.0.0.0 – 239.255.255.255 (Class D) is for multicast.
- 240.0.0.0 – 255.255.255.255 (Class E) is reserved/experimental.
3. Private IP Address Ranges (RFC 1918)
These ranges are widely used in enterprises and cloud environments:
| Class | Private Range | CIDR |
|---|---|---|
| Class A | 10.0.0.0 – 10.255.255.255 | /8 |
| Class B | 172.16.0.0 – 172.31.255.255 | /12 |
| Class C | 192.168.0.0 – 192.168.255.255 | /16 |
These ranges are non-routable on the public internet and are typically used for internal networks, VLANs, firewalls, VPNs, and subnetting.
4. Subnet Ranges for Each IPv4 Class
4.1 Subnet Ranges for Class A Networks
A Class A network (/8) supports extremely large subnets. When subnetted, the ranges look like:
| CIDR | Subnet Mask | Number of Subnets | Hosts per Subnet |
|---|---|---|---|
| /9 | 255.128.0.0 | 2 | 8,388,606 |
| /10 | 255.192.0.0 | 4 | 4,194,302 |
| /12 | 255.240.0.0 | 16 | 1,048,574 |
| /16 | 255.255.0.0 | 256 | 65,534 |
| /24 | 255.255.255.0 | 65,536 | 254 |
Example:
10.0.0.0/16 produces subnets like:
- 10.0.0.0 – 10.0.255.255
- 10.1.0.0 – 10.1.255.255
- 10.2.0.0 – 10.2.255.255
4.2 Subnet Ranges for Class B Networks
Class B (/16) is common for medium-to-large enterprise networks.
| CIDR | Subnet Mask | Number of Subnets | Hosts per Subnet |
|---|---|---|---|
| /17 | 255.255.128.0 | 2 | 32,766 |
| /18 | 255.255.192.0 | 4 | 16,382 |
| /19 | 255.255.224.0 | 8 | 8,190 |
| /20 | 255.255.240.0 | 16 | 4,094 |
| /22 | 255.255.252.0 | 64 | 1,022 |
| /24 | 255.255.255.0 | 256 | 254 |
Example:
172.16.0.0/20 produces subnets at increments of 16 in the third octet:
- 172.16.0.0
- 172.16.16.0
- 172.16.32.0
- 172.16.48.0
4.3 Subnet Ranges for Class C Networks
Class C (/24) is heavily used in smaller LANs, VLANs, and firewall segmentation.
| CIDR | Subnet Mask | Number of Subnets | Hosts per Subnet |
|---|---|---|---|
| /25 | 255.255.255.128 | 2 | 126 |
| /26 | 255.255.255.192 | 4 | 62 |
| /27 | 255.255.255.224 | 8 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 32 | 6 |
| /30 | 255.255.255.252 | 64 | 2 |
Example:
192.168.1.0/27 produces subnets in increments of 32:
- 192.168.1.0 – 192.168.1.31
- 192.168.1.32 – 192.168.1.63
- 192.168.1.64 – 192.168.1.95
- 192.168.1.96 – 192.168.1.127
5. Why Subnetting Matters in Security and Network Architecture
Subnetting is not only a logical division technique; it directly supports:
5.1 Network Security
- Reduces attack surface by isolating devices
- Simplifies firewall rules and segmentation
- Supports Zero Trust designs
- Minimizes broadcast-domain attacks
5.2 Compliance & Governance
- Required for PCI DSS network segmentation
- Supports SOC 2 logical access and security controls
- Helps in ensuring data flow isolation
5.3 Performance and Scalability
- Reduces ARP broadcasts
- Improves routing efficiency
- Supports scalable enterprise-scale designs
6. Practical Tips for Choosing Subnet Ranges
- Use private IPs as per RFC 1918 unless a public block is required.
- Segment by function, e.g.,
- Web servers
- DB servers
- Admin networks
- Client VLANs
- Avoid excessively large subnets to minimize broadcast traffic.
- Document and version-control network IP addressing using Git or CMDB.
- Use CIDR blocks that align with your future growth, not just current needs.
- Apply route summarization where possible to reduce routing table size.
7. Conclusion
Understanding subnet ranges across IPv4 Class A, B, and C networks is fundamental to secure architecture, segmentation, and network design. Whether deploying enterprise infrastructure, configuring firewalls, or designing cloud networks, accurate subnetting ensures predictable behavior, improved security, and scalable performance.
This structured guide provides the essential class ranges, subnet masks, and practical examples needed for real-world implementation.
Frequently Asked Questions (FAQs)
What are IPv4 class ranges?
IPv4 class ranges are predefined address blocks – Class A, B, C, D, and E used in the original classful addressing system for organizing network allocation.
What is the default subnet mask for Class A?
The default subnet mask for Class A is 255.0.0.0 (CIDR /8).
What is the default subnet mask for Class B?
Class B uses 255.255.0.0 (CIDR /16).
What is the default subnet mask for Class C?
Class C uses 255.255.255.0 (CIDR /24).
What are the private IP ranges defined in RFC 1918?
The private ranges are
– 10.0.0.0/8
– 172.16.0.0/12
– 192.168.0.0/16
What is CIDR notation in IPv4 subnetting?
CIDR notation expresses a subnet mask as a prefix length (e.g., /24) representing the number of network bits.
How many hosts are available in a /24 subnet?
A /24 subnet provides 254 usable hosts.
How many hosts does a /27 subnet support?
A /27 subnet supports 30 usable hosts.
Why is classful addressing still relevant today?
It remains relevant for certification exams, legacy systems, network documentation, and public IP allocation concepts.
What is a subnet range?
A subnet range is the block of IP addresses defined by a network prefix and subnet mask within a larger IP network.
What is the smallest usable IPv4 subnet?
The smallest functional subnet is /30, providing 2 usable addresses, typically for point-to-point links.
What is the largest subnet in private networks?
10.0.0.0/8 is the largest private subnet, offering over 16 million host addresses.
How do Class A subnets work?
Class A networks start with 1 – 126 and can be subnetted into smaller CIDR blocks such as /9, /10, /12, etc.
How do Class B subnets work?
Class B ranges (128 – 191) use /16 by default but are commonly divided into /17, /18, /20, or /22 blocks.
How do Class C subnets work?
Class C ranges (192 – 223) use /24 and are frequently subdivided into /25, /26, /27, /28, /29, and /30 networks.
What is VLSM in subnetting?
Variable Length Subnet Masking (VLSM) allows subnetworks of different sizes within the same address space.
Why is subnetting important for cybersecurity?
Subnetting isolates network zones, reduces lateral movement risk, and supports micro-segmentation and firewall enforcement.
How does subnetting improve network performance?
It reduces broadcast traffic, supports better routing summarization, and enables efficient resource allocation.
What is a broadcast address in a subnet range?
The broadcast address is the last IP in a subnet and is used for broadcasting packets to all devices in that subnet.
What is a network address in subnetting?
The network address is the first IP in a subnet and identifies the subnet itself, not a host.
How is the subnet increment calculated?
The increment is derived from the subnet mask’s block size per octet (e.g., /27 → 256 – 224 = 32).
What tools can help calculate subnet ranges?
Tools include: ipcalc, Subnet calculators (online), Cisco Packet Tracer, Linux ip and ifconfig utilities
What are Class D and Class E address ranges?
Class D (224–239): multicast
Class E (240–255): reserved/experimental
What is the purpose of the 127.0.0.0 range?
127.0.0.0/8 is the loopback address range used for local system testing.
Why is the first and last IP in a subnet not assignable?
The first IP is reserved as the network identifier, and the last IP is reserved for broadcast.
Can IPv4 subnets overlap?
Overlapping subnets cause routing conflicts and must be avoided in production networks.
What is the difference between subnetting and supernetting?
Subnetting creates smaller networks from a larger block.
Supernetting aggregates smaller networks into a larger block (route summarization).
How does subnetting support Zero Trust security?
It enforces strict segmentation, limits east-west movement, and helps build policy boundaries between services.
Can I use public IP ranges for internal subnets?
No. Using public IPs internally can cause routing conflicts and violates best practices.
Is IPv4 subnetting still important with IPv6 adoption?
Yes. IPv4 remains widely used in enterprises, cloud, ISPs, and legacy systems, making subnetting a required skill.