Discovery · Lesson 11

Why did CIDR replace IPv4 address classes?

IPv4 classes A, B, and C used fixed network sizes, while CIDR lets operators choose a prefix that fits the actual address block.

The historical model

Classes made the first network boundary predictable.

Classful IPv4 addressing used leading bits to select a default network size. Class A used a default /8 network, Class B a /16, and Class C a /24.

That simple model became wasteful as networks grew in many different sizes. An organisation needing more than a /24 but far less than a /16 could not choose a right-sized classful block.

Classes are historical terminology; CIDR is the operational model.You may still hear “Class C” in older documentation, but modern routing and allocation use explicit prefix lengths such as /20 or /27.

Build the model

Five historical ranges

01

Class A

First octet 1–126 in the traditional usable range; default /8 network.

02

Class B

First octet 128–191; default /16 network.

03

Class C

First octet 192–223; default /24 network.

04

Class D

224–239, reserved for IPv4 multicast groups rather than host subnets.

05

Class E

240–255, reserved for experimental or future use in the historical model.

Compare with CIDR

Prefix length gives the network the right size

CIDR separates the network prefix length from the old class labels. A /20 contains 4,096 IPv4 addresses, while a /27 contains 32, so allocation can better match the need and routes can be aggregated.

Historical IPv4 classes A through E compared with CIDR blocks using /20 and /27 prefixes.
Classful ranges were fixed; CIDR chooses an explicit prefix length.

Knowledge check

Test your classful and CIDR model

Answer six questions.

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