IP Address Classes: The Original System

Before CIDR, the internet used "classes" to size networks. Let's explore this historical system.

The Five IP Classes

Class	First Octets	Default Mask	Total Hosts
A	1-126	/8	16M+
B	128-191	/16	65K
C	192-223	/24	254
D	224-239	-	Multicast
E	240-255	-	Reserved

First octet: 1-126
Default mask: 255.0.0.0 (/8)
Examples: 3.0.0.0 (GE), 12.0.0.0 (AT&T)

Only 126 Class A networks existed. Each had millions of addresses. Companies like GE got entire Class A!

First octet: 128-191
Default mask: 255.255.0.0 (/16)
Example: 128.2.0.0 (MIT)

About 16,000 Class B networks. Each has 65,534 addresses.

First octet: 192-223
Default mask: 255.255.255.0 (/24)
Example: 192.168.1.0 (common home/office)

Most common today. 254 addresses per network.

Note: Class A 127.x.x.x is reserved for "localhost" — your own computer.

Classes were too rigid:

CIDR replaced classes in 1993. But you'll still see "Class A/B/C" in discussions.