Network Classes
Note - Class-based network numbers are no longer available from the IANA, though many older
networks are still class-based.
This section provides details about IPv4 network classes. Each class uses the 32-bit
IPv4 address space differently, providing more or fewer bits for the network part
of the address. These classes are class A, class B, and class C.
Class A Network Numbers
A class A network number uses the first 8 bits of the
IPv4 address as its “network part.” The remaining 24 bits contain the host part
of the IPv4 address, as the following figure illustrates.
Figure 10-3 Byte Assignment in a Class A Address
The values that are assigned to the first byte of class A
network numbers fall within the range 0–127. Consider the IPv4 address 75.4.10.4. The value
75 in the first byte indicates that the host is on a
class A network. The remaining bytes, 4.10.4, establish the host address. Only the first
byte of a class A number is registered with the IANA. Use
of the remaining three bytes is left to the discretion of the owner
of the network number. Only 127 class A networks exist. Each one of
these numbers can accommodate a maximum of 16,777,214 hosts.
Class B Network Numbers
A class B network number uses 16 bits for the network number
and 16 bits for host numbers. The first byte of a class B
network number is in the range 128–191. In the number 172.16.50.56, the first two
bytes, 172.16, are registered with the IANA, and compose the network address. The
last two bytes, 50.56, contain the host address, and are assigned at the
discretion of the owner of the network number. The following figure graphically illustrates
a class B address.
Figure 10-4 Byte Assignment in a Class B Address
Class B is typically assigned to organizations with many hosts on their networks.
Class C Network Numbers
Class C network numbers use 24 bits for the network number and 8
bits for host numbers. Class C network numbers are appropriate for networks with
few hosts—the maximum being 254. A class C network number occupies the first
three bytes of an IPv4 address. Only the fourth byte is assigned at
the discretion of the network owners. The following figure graphically represents the bytes
in a class C address.
Figure 10-5 Byte Assignment in a Class C Address
The first byte of a class C network number covers the range
192–223. The second and third bytes each cover the range 1– 255. A
typical class C address might be 192.168.2.5. The first three bytes, 192.168.2, form
the network number. The final byte in this example, 5, is the
host number.
