Goodbye Classes

In the early internet, the first octet alone decided how big your network was. Class A (first octet 1-126) got a /8, Class B (128-191) a /16, and Class C (192-223) a /24. No other sizes existed.

That wasted addresses on a heroic scale: a company with 300 machines was too big for a Class C and got a Class B with 65,534 addresses. So in 1993 the internet switched to CIDR (Classless Inter-Domain Routing), where any prefix length is allowed.

Classes are dead for routing, but the vocabulary survives in job interviews and old documentation, so you still need to recognize them.