X7968r
User’s Guide
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Appendix C - IP Addresses, Network
Masks, and Subnets
IP Addresses
Note
This section refers only to IP addresses for IPv4 (version 4 of the Internet
Protocol). IPv6 addresses are not covered.
This section assumes basic knowledge of binary numbers, bits, and bytes.
IP addresses, the Internet's version of telephone numbers, are used to identify individual nodes
(computers or devices) on the Internet. Every IP address contains four numbers, each from 0 to 255
and separated by dots (periods), e.g. 20.56.0.211. These numbers are called, from left to right, field1,
field2, field3, and field4.
This style of writing IP addresses as decimal numbers separated by dots is called dotted decimal
notation. The IP address 20.56.0.211 is read "twenty dot fifty-six dot zero dot two-eleven."
Structure of an IP address
IP addresses have a hierarchical design similar to that of telephone numbers. For example, a 7-digit
telephone number starts with a 3-digit prefix that identifies a group of thousands of telephone lines,
and ends with four digits that identify one specific line in that group.
Similarly, IP addresses contain two kinds of information:
Network ID
Identifies a particular network within the Internet or intranet
Host ID
Identifies a particular computer or device on the network
The first part of every IP address contains the network ID, and the rest of the address 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 organizations 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.
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