Classful vs Classless IP Addressing
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The Original Approach: Classful Addressing
Early IPv4 addressing divided the address space into fixed classes, each with a predetermined network/host split.
| Class | Address range | Default network portion |
|---|---|---|
| A | 1.0.0.0 - 126.255.255.255 | First 8 bits (huge networks, few allowed) |
| B | 128.0.0.0 - 191.255.255.255 | First 16 bits |
| C | 192.0.0.0 - 223.255.255.255 | First 24 bits (small networks, many allowed) |
Why Classful Addressing Was Wasteful
An organization needing 300 addresses had no in-between option: a Class C network only offered 254 usable addresses (too few), while a Class B network offered over 65,000 (wasting the vast majority). This rigid, one-size-fits-few structure wasted enormous numbers of addresses across the internet.
The Fix: Classless Addressing (CIDR)
Classless Inter-Domain Routing (CIDR) removed the fixed class boundaries entirely, letting the network/host split happen at any bit position, not just at 8, 16 or 24. This is written using a slash notation, like /26, indicating exactly how many bits belong to the network portion.
192.168.1.0/26 -> first 26 bits are network, remaining 6 bits are host
(allows exactly 62 usable host addresses - a much closer fit)Why CIDR Became the Standard
CIDR lets an organization request exactly the address block size it needs, rather than being forced into oversized or undersized fixed classes. This single change dramatically slowed the exhaustion of the IPv4 address space and remains the addressing scheme used everywhere today.