SMC Networks ADSL Barricade SMC7204BRA - Manual (Page 166)

Network classes
169
Network classes
The three commonly used network classes are A, B and C.
(There is also a class D but it has a special use beyond the
scope of this discussion.) These classes have different uses
and characteristics.
Class A networks are the Internet's largest networks, each with
room for over 16 million hosts. Up to 126 of these huge networks
can exist, for a total of over 2 billion hosts. Because of their huge
size, these networks are used for WANs and by organizations
at the infrastructure level of the Internet, such as your ISP.
Class B networks are smaller but still quite large, each capable
to hold over 65,000 hosts. There can be up to 16,384 class B
networks in existence. A class B network might be appropriate
for a large organization such as a business or government
agency.
Class C networks are the smallest, only able to hold 254 hosts at
most, but the total possible number of class C networks exceeds
2 million (2,097,152 to be exact). LANs connected to the Internet
are usually class C networks.
Some important notes regarding IP addresses:
•
The class can be determined easily from field1:
field1 = 1-126: Class A
field1 = 128-191: Class B
field1 = 192-223: Class C
(field1 values not shown are reserved for special uses)
•
A host ID can have any value except all fields set to 0
or all fields set to 255, as those values are reserved for
special uses.