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Helpdesk ticket: 'printer at 192.168.1.50 stopped working after we added a second office.' You check: both offices use 192.168.1.0/24. Same numbers, two places = conflict. This module is why every device needs a unique address — and how to carve them without collisions.
Classful addressing is dead — CIDR killed it in 1993. But the exam still asks about it, and more importantly, the reason classful failed explains why subnet masks exist at all. Learn it as history with a purpose, not as trivia.
An IPv4 address is a 32-bit binary number. Nobody wants to read `11000000.10101000.00000001.00000001`, so we write it as four decimal octets separated by dots: `192.168.1.1`. Each octet is 8 bits, values 0–255.
When you see `10.20.30.40`, the machine sees `00001010.00010100.00011110.00101000`. Every subnetting calculation you'll do for the rest of your career happens in that binary form, whether you do it in your head or with a calculator.
| Class | First octet range | Default mask | Purpose |
|---|---|---|---|
| A | 1–126 | /8 (255.0.0.0) | Very large networks |
| B | 128–191 | /16 (255.255.0.0) | Medium networks |
| C | 192–223 | /24 (255.255.255.0) | Small networks |
| D | 224–239 | N/A | Multicast |
| E | 240–255 | N/A | Experimental / reserved |
Class A excludes `127` — that entire range is reserved for loopback. `127.0.0.1` is the classic loopback address, and every address in `127.0.0.0/8` behaves the same on a host.
The class boundaries are decided by the leading bits of the first octet. Class A starts with `0`, Class B with `10`, Class C with `110`, Class D with `1110`, and Class E with `1111`. That's where the ranges come from — the range math is a consequence of the leading-bit encoding.
Classful addressing forces a network to be one size: a Class B is always 65,534 hosts. A company needing 500 hosts had to burn an entire Class B (wasting 64,000 addresses) or manage four Class C networks glued together. Neither is acceptable at scale.
By the early 1990s the internet was running out of Class B networks, and the global routing table was exploding as every Class C got its own entry. CIDR — Classless Inter-Domain Routing — fixed both problems by allowing masks of any length, not just /8, /16, /24.
A junior engineer is auditing an old network diagram and sees these addresses assigned to hosts: 172.16.5.10, 200.100.50.25, 10.200.30.40, and 192.168.10.1. He needs to know each one's legacy class and default mask.
Select the safest next action.
Classful addressing won't come up in your day job — every network since 1995 has used CIDR. But knowing the class table cold is a five-minute investment that pays off on the exam and in any legacy documentation you'll read.
Next lesson covers public vs. private addressing and the special-use ranges you need to recognize on sight.
Finish the lesson once you have worked through the material. This awards ★ 45 XP.