Internet Security Router User
’s Manual
Chapter 14. IP Addresses, Network Masks, and Subnets
135
14 IP Addresses, Network Masks, and Subnets
14.1 IP Addresses
Note
This section pertains 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. For details on this subject, see Appendix 13.
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."
14.1.1 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). Table 14.1 shows the
structure of an IP address.
Table 14.1. IP Address structure
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)
14.2 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.