Class A B and C IP Networks
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The IPv4 address space includes all possible combinations of numbers for a 32-bit IPv4
address. Literally 232 different values exist with a 32-bit number, for more than 4 billion
different numbers. With DDN values, these numbers include all combinations of
the values 0 through 255 in all four octets: 0.0.0.0, 0.0.0.1, 0.0.0.2, and all the way up to
IP standards first subdivide the entire address space into classes, as identified by the value
of the first octet. Class A gets roughly half of the IPv4 address space, with all DDN numbers
that begin with 1–126, as shown in Figure 4-6. Class B gets one-fourth of the address
space, with all DDN numbers that begin with 128–191 inclusive, and Class C gets oneeighth
of the address space, with all numbers that begin with 192–223.
Figure 4-6 also notes the purpose for the five address classes. Classes A, B, and C define
unicast IP addresses, meaning that the address identifies a single host interface. Class D
defines multicast addresses, used to send one packet to multiple hosts. Class E originally
defined experimental addresses. (Class E addresses are no longer defined as experimental,
and are simply reserved for future use.)
IPv4 standards also subdivide the Class A, B, and C unicast classes into predefined IP networks.
Each IP network makes up a subset of the DDN values inside the class.
Figure 4-6 Division of the Entire IPv4 Address Space by Class
IPv4 uses three classes of unicast addresses so that the IP networks in each class can
be different sizes, and therefore meet different needs. Class A networks each support a
very large number of IP addresses (more than 16 million host addresses per IP network).
However, because each Class A network is so large, Class A holds only 126 Class A networks.
Class B defines IP networks that have 65,534 addresses per network, but with
space for more than 16,000 such networks. Class C defines much smaller IP networks,
with 254 addresses each, as shown in Figure 4-7.
Figure 4-7 Size of Network and Host Parts of Class A, B, and C Addresses
Figure 4-7 shows a visual perspective, as well as the literal numbers, for all the Class A, B,
and C IPv4 networks in the entire world. The figure shows clouds for IP networks. It, of
course, does not show one cloud for every possible network, but shows the general idea,
with a small number of large clouds for Class A and a large number of small clouds for