TCP/IP Tutorial and Technical Overview - IBM Redbooks

There are five classes of IP addresses. They are shown in Figure 3-1.
Class A
Class B
Class C
Class D
Class E
Figure 3-1 IP: Assigned classes of IP addresses
Where:
Class A addresses These addresses use 7 bits for the and 24 bits
for the portion of the IP address. This allows for
2 7 -2 (126) networks each with 2 24 -2 (16777214) hosts—a
total of more than 2 billion addresses.
Class B addresses These addresses use 14 bits for the and 16
bits for the portion of the IP address. This allows
for 2 14 -2 (16382) networks each with 2 16 -2 (65534)
hosts—a total of more than 1 billion addresses.
Class C addresses These addresses use 21 bits for the and 8 bits
for the portion of the IP address. That allows for
2 21 -2 (2097150) networks each with 2 8 -2 (254) hosts—a
total of more than half a billion addresses.
Class D addresses These addresses are reserved for multicasting (a sort of
broadcasting, but in a limited area, and only to hosts
using the same Class D address).
Class E addresses These addresses are reserved for future or experimental
use.
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