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Networking Fundamentals · Foundational

Classful Addressing

The original IPv4 scheme that divided the address space into fixed classes A through E based on the leading bits.

Definition

Under classful addressing, the first octet determined the default mask: Class A /8, Class B /16, Class C /24, with Class D for multicast and Class E reserved. It wasted addresses and was replaced by CIDR, but class ranges and default masks still appear on exams.

Example

Under classful rules, 150.10.1.1 is Class B with default mask 255.255.0.0.

Exam tip

First-octet ranges: A 1-126, B 128-191, C 192-223, D 224-239, E 240-255 (127 is loopback).

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