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Network features 437

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

client can also have a list of parameters it would like to acquire and can

request them from the DHCP server by including the Parameter Request

List option (Option Code 55) in the DHCPDISCOVER message.

When the DHCP server sees the broadcast, it responds by broadcasting

its own DHCP message. The server, since it knows more about the

network, is able to fill in most of the information in the message. For

example, information such as the server IP address and gateway IP

address are included in their respective fields. Since the client does not

have an IP address yet, the server uses the client’s MAC address to

uniquely identify it. When the client sees the broadcast, it matches its MAC

address against the one in the message.

DHCP options

DHCP options are the sub-fields of the Options field. They carry additional

network configuration information requested by the client such as the IP

address lease length and the subnet mask.

Each DHCP option has an associated option code and a format for

carrying data. Usually the format is as follows:

Option code Length Data

There are two categories of DHCP options: standard and non-standard.

The standard options are predefined by the industry. The non-standard

options are user-defined to fit the needs of a particular vendor or site.

Nortel Communication Server 1000

IP Phones Fundamentals

NN43001-368 05.06 30 April 2010