ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 289)

It can also be started automatically by creating a script in the /etc/rc directory

and by incorporating the following command:

/usr/sbin/dhcpd eth0

NAT (Network Address Translation)

NAT is a technique used for connecting several computers to the Internet on the

same IP address. NAT has been and still is widely used for compensating for the

small number of available IP addresses.

Let us suppose a PLC network in which a PLC modem-router is connected to the

Internet, as illustrated in Figure 11.18.

The network computers can only gain access to the Internet if the Internet

modem or another entity in the network incorporates NAT routing functions and is

connected to the Internet. Most of them incorporate the NAT.

The NAT routing makes it possible to use only one routable address over the

Internet for a group of computers having non-routable, fixed private addresses.

When a computer sends data not intended for the local area network, the NAT

router—the Internet modem in this case—replaces the IP address of the sender by the

connection IP address given by the Internet access provider (@net on the figure). At

the same time, the Internet modem writes the connection information (IP address of

the sender, protocol used) to a translation table.

When the Internet modem receives data from the Internet, it checks the data

receiver in its translation table by comparing the type of received data with the infor-

mation contained in the table. Once the receiver is found, the IP @net address is

replaced by that of the receiver. In this way, all the network computers use the same

IP address for gaining access to the Internet.

The NAT can filter the incoming packets and avoid external attacks with this

addressing scheme. If the connection is not initiated by the computers, the external

packets cannot be processed by the NAT router.

NAT Configuration

Unlike the DHCP server, the NAT depends on the kernel used, 2.2 or 2.4/2.6. In

both cases, and as for the DHCP server, it is possible either to start the NAT manu-

270 PLC for Businesses

Figure 11.18 PLC network connected to the Internet

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 289