IP Address Classes Explained: A, B, C, D and E
The original IPv4 scheme split addresses into five classes (A–E) by their leading bits. Though CIDR has replaced rigid classes in practice, the ranges still appear in exams and shape how we talk about addresses.
The five classes
| Class | First octet | Default mask | Use |
|---|---|---|---|
| A | 1–126 | /8 | Very large networks |
| B | 128–191 | /16 | Medium networks |
| C | 192–223 | /24 | Small networks |
| D | 224–239 | — | Multicast |
| E | 240–255 | — | Experimental |
(127 is reserved for loopback.)
Why classes gave way to CIDR
Classful allocation was wasteful — an organisation needing 300 addresses had to take a whole Class B (65,000). CIDR replaced classes with flexible slash notation so networks can be any size, and it enabled route summarisation. Classes remain useful vocabulary but not how modern addressing is actually assigned.
Frequently asked questions
What are the IP address classes?
Class A (1–126), B (128–191), C (192–223) for regular addressing, D (224–239) for multicast, and E (240–255) experimental. 127 is loopback.
What is the default subnet mask for Class C?
/24, or 255.255.255.0, giving 254 usable host addresses per network.
Are IP classes still used?
Not for allocation — CIDR replaced classful addressing — but the class ranges still appear in exams and networking discussion.
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