If you've encountered the terms "Class A," "Class B," or "Class C" IP addresses, you've run into a system that mostly stopped being used for real network design decades ago — but understanding it briefly still helps make sense of some older documentation and networking terminology that hasn't gone away.
What the classes originally meant
The original IPv4 addressing scheme divided the address space into fixed-size classes based on the first few bits of an address:
- Class A (1.0.0.0–126.255.255.255) — a small number of very large networks, each with room for over 16 million addresses. Reserved for the largest early organizations.
- Class B (128.0.0.0–191.255.255.255) — mid-sized networks, each with room for about 65,000 addresses.
- Class C (192.0.0.0–223.255.255.255) — smaller networks, each with 256 addresses, intended for smaller organizations.
Why it was replaced
Fixed class sizes were wasteful — an organization needing 300 addresses had to be allocated an entire Class B block (65,000+ addresses) because there was no size in between, wasting the vast majority of it. As the internet grew, this inefficiency became untenable, and the industry moved to CIDR (Classless Inter-Domain Routing) in the 1990s, which allows address blocks of any size via prefix notation like /24 or /16, allocated much more precisely to actual need.
Where the old terminology still shows up
You'll still see "Class C" used informally to mean "a /24 block" in some documentation and casual conversation, even though the formal class system itself is retired. The reserved private address ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16) also roughly trace their origins back to the old class boundaries, which is part of why they look the way they do.
FAQ
Do I need to know address classes to understand my own network?
No — modern networking documentation and router interfaces use CIDR notation almost universally. The class system is background history, not something you need for everyday troubleshooting.