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2-15 Supplement
2
42
■ IP address
The IP address is a unique number assigned to each device to identify that device on the network
and prevent the same number from being used by different devices. (More accurately, an IP ad-
dress is assigned to each network interface, since once PC may sometimes be installed with mul-
tiple network interfaces.) In a TCP/IP protocol, the data transmit source and destination are speci-
fied by this IP address. The IP address consists of 32 bits (4 bytes) so can be expressed with this
number without changes, however it is normally expressed as a decimal number separated by
periods (.) at each byte (in other words, four sets of numbers separated by periods). An IP address
of 0xC0A80002, for example, is normally expressed as 192.168.0.2
The IP address is actually comprised of 2 address sections. One section is the network address.
The network address is the address of the network itself. The other section is the host address
section. The host address is an address for identifying each device on that network. The IP ad-
dress, as shown below, uses the first through the Nth bits as the network address, and the N+1 bit
through 32nd bit as the host address. (The value of N is determined by the subnet mask.)
IP address
In an IP address of 192.168.0.2, for example, if the N value (network length) is 24 bits, then the
network address section is 192.168.0, and the host address section is 2. Generally, in a network
address, the host address section is 0 and the network length is listed behind the address. In the
above example, this would be shown as 192.168.0.0/24.
One network can be connected with as many devices as there are addresses to identify them.
However, host address bits having all zeroes (0), or all ones (1) are reserved and so cannot be used.
In the above example, though the host address can identify 256 devices, the numbers 0 and 255
cannot be used so the maximum number of devices that can actually be connected is 254.
192.168.0.0/24
192.168.0.1
192.168.0.100
192.168.0.254
Network A
Any company (organization) can freely select a host address but when attempting to connect their
network to the Internet, that company (organization) cannot select the network address on their
own. An application to acquire a network address must be made to the NIC (in Japan, JPNIC). If
connecting one's network to the Internet is not necessary, then any company can freely select a
network address, as well as a host address.
If there is no need to connect to the Internet, then use of the following addresses is allowed.
10.0.0.0 through 10.255.255.255
172.16.0.0 through 172.31.255.255
192.168.0.0 through 192.168.255.255
An address acquired by making application to NIC on the other hand is referred to as a global address.
1
N N+1
Network address section Host address section
Maximum of
254 devices
connected by
router
(1 unit of class A)
(16 units of class B)
(256 units of class C)
32
192.168.1.0/24
192.168.1.1
Maximum of
192.168.1.100
254 devices
192.168.1.254
Network B
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