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CHAPTER2 Ethernet Unit
■ 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 ap-
plying 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 effi-
ciently 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 manufacturing 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 ad-
dress 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 con-
trolled based on a sophisticated process. The router sends data with an external desti-
nation 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 set-
ting allows data to be correctly sent and received by each device on the network.