ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka stáhnout pdf (Strana 260)

Once these parameters have been received, the computer can dialogue freely

with other computers on the network or have access to the Internet if there is a con-

nection sharing scheme. This is a user-transparent mechanism that does not take

more than one second.

Another characteristic feature of DHCP is the lease. As we explained above, the

parameters given to a network station are valid for a given period of time only. This

lease is negotiated between the computer and the server when parameters are

requested. When this lease expires, it can still be renegotiated by the computer.

Dynamic Configuration of a DHCP Client

The dynamic configuration of a computer that connects to a DHCP server takes

place in four phases, as illustrated in Figure 10.26:

•

When a DHCP client accesses a network, no address is allocated to him and

his IP address is 0.0.0.0.

•

In order to configurate himself, the client sends a DHCP DISCOVER request

in broadcast mode – with IP address 255.255.255.255 – over the network in

which he inserts his MAC address.

MAC Address

The MAC address is a fixed address assigned to each Ethernet board of the terminals con-

nected to the PLC network.

•

The DHCP server replies with a DHCP OFFER always sent in broadcast

mode since the client does not have an IP address yet. The DHCP OFFER is

made up of the client’s MAC address, the lease time, and the server IP address.

It is possible to have several DHCP servers, but we only use one within the

framework of this book.

•

If the client accepts this offer, he sends a DHCP REQUEST in order to receive

the parameters.

•

The server sends a DHCP PACK confirming the client’s acceptance.

Configuring an Internet Gateway 241

Figure 10.26 Dynamic configuration of a computer via the DHCP

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ZyXEL Communications PLA-470 V2 - V3.0.5 Instalační příručka Strana 260