Dhcp Options - Nortel Meridian ITG Line 2.0 Description, Installation And Operation Manual

Page 285 of 302
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 may 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 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.
Using this method, the server and client can supply or receive information
through the exchange of their DHCP messages.

DHCP Options

DHCP Options are the sub fields of the Options field. They carry additional
network configuration information requested by the client such as IP address
lease length and 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 while non-standard options
are user-defined to fit the needs of a particular vendor or site.
There are a total of 255 DHCP option codes where option codes 0 and 255 are
reserved, 1-77 are predefined, 1-254 can be used for Vendor Specific options
and 128-254 are designated for Site Specific options. This arrangement
allows for future expansion and is to be used as a guideline for choosing
option codes.
ITG Line 2.0/i2004 Internet Telephone Description, Installation, and Operation