Avaya 1110 Fundamentals page 384

Features
DHCP Message Types
DHCPDECLINE
DHCPRELEASE
DHCP message format
The DHCP message format shown
common to all DHCP messages. Each message consists of 15 fields: 14 fixed-length fields
and one variable length field. The 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 68: 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 client can also have a list of parameters it would like to acquire and can request them
384 IP Deskphones Fundamentals
Notifies server that offer is unsatisfactory and will not
be accepted.
Notifies server that IP address is no longer needed.
inFigure 68: DHCP message format
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Description
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February 2013