System information

418 Features

fixed-length fields must be the specified number of bytes, as indicated in

the brackets. If there is not enough data, or there is no data at all, zeros

are used to fill in the extra spaces.

Figure 76

DHCP message format

The Options field is the only field with a variable length. It is optional,

but very important, as it transports additional network configuration

parameters. The DHCP options are the actual subfields that are used in

this project.

DHCP message exchange

For a client to receive services from a DHCP server, an exchange of

DHCP messages between the client and server must take place. The

sequence and types of DHCP message exchanged can differ, but the

mechanism of acquiring and supplying information remains the same.

Usually the client initiates the exchange with a DHCP message broadcast.

Using a broadcast enables the client to send messages to all servers on

the network without having an associated IP address. The broadcast is

local to the LAN, unless a DHCP relay agent is present to forward the

packet.

At this point, the client has no information about the server or the IP

address it is going to receive (unless it is requesting a renewal), so the

fields in the DHCP message are empty. However, the client knows its own

MAC address and includes it in the Client hardware address field. The

Nortel Communication Server 1000

IP Phones Fundamentals

NN43001-368 05.02 26 May 2009