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■ Subnet mask
The subnet mask is used to separate the IP address into a network address section and a host
address section. The network address bit is set to 1, and the host address bit is set to 0.
The subnet mask, just like the IP address is expressed as a decimal number of 32 bits (4 bytes)
with each byte separated by a period (or four sets of numbers separated by periods). So if the
subnet mask is 255.255.255.0, then the network address section is 24 bits.
A company (organization) is generally assigned only one network address when applying to the
NIC for an IP address. The company making the application falls within one of classes A, B or C
depending on the scale of the company. Class B for example, has a network length of 16 bits and
can be assigned a network allowing connection of up to 65533 devices. However, unless changes
are made, this network cannot efficiently perform the required managing and processing tasks. So
such a network is normally set with subnet masks to divide it into an appropriate number of
smaller networks. When a class B network for example, is set with a subnet mask of 255.255.255.0,
a total of 256 settings can be made allowing up to 254 devices to be connected.
■ MAC address (Media access control address)
The MAC address, also called the Ethernet address is a hardware type identification number (6
bytes) set in each network interface. The MAC address is set in each device during the manufac-
turing stage and therefore does not have to be set by the user.
Each device in the Ethernet system is identified by means of this MAC address. In other words,
the IP address is automatically converted to a MAC address, even when communicating by means
of a TCP/IP protocol.
Basically the user does not normally have to even be aware of the MAC address. However, if there
are communication problems, then the interrelation of the IP address and MAC address can be
checked to find out if the cause of the problem is overlapping (identical) IP addresses.
■ HUB
A HUB is a device used for connecting devices such as PCs by way of a 10BASE-T network. The
HUB has multiple ports that allow connecting modular jacks and twisted pair cables fitted with
these modular jacks connect to the HUB from each device.
The HUB may have different type connectors depending on whether the HUB is for 10BASE-2 or
10BASE-5. Various types of networks can be constructed by means of these HUBs.
■ Router
The router is a device for mutually connecting networks together. The router is controlled based
on a sophisticated process. The router sends data with an external destination from an internal
network to an external network, and sends data received from an external network, to an internal
network. Designated data is discarded in a filtering process to help maintain network safety.
The router IP address is set as the gateway address in each network device. This setting allows
data to be correctly sent and received by each device on the network.
2-15 Supplement
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