Similarly, IP addresses contain two kinds of information:
NetworkID•
Identies a particular network within the Internet or intranet
HostID•
Identies a particular computer or device on the network
The rst part of every IP address contains the network ID, and the rest of the ad-
dress contains the host ID. The length of the network ID depends on the network’s
class (see following section). The table below shows the structure of an IP address.
Field1 Field2 Field3 Field4
Class A Network ID Host ID
Class B Network ID Host ID
Class C Network ID Host ID
Here are some examples of valid IP addresses:
Class A: 10.30.6.125 (network = 10, host = 30.6.125)
Class B: 129.88.16.49 (network = 129.88, host = 16.49)
Class C: 192.60.201.11 (network = 192.60.201, host = 11)
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 organi-
zations at the infrastructure level of the Internet, such as your ISP.
Class B networks are smaller but still quite large, each able 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 eld1: •
eld1 = 1-126: Class A
eld1 = 128-191: Class B
eld1 = 192-223: Class C
(eld1 values not shown are reserved for special uses)
A host ID can have any value except all elds set to 0 or all elds set to 255, •
as those values are reserved for special uses.
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