System information

Network features 419
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.
There are a total of 255 DHCP option codes where option codes 0 and
255 are reserved, 1 to 77 are predefined, 1 to 254 can be used for Vendor
Specific Options, and 128 to 254 are designated for Site Specific Options.
This arrangement enables future expansion and is used as a guideline
for choosing option codes.
Vendor Specific/Encapsulated option
The Vendor Specific DHCP options are vendor-defined options for carrying
vendor-related information. It is possible to override predefined standard
options; however, doing so can cause conflict when used with components
that follow the industry standard.
A useful option is the standard Vendor Encapsulated option – code 43. It
is used to encapsulate other DHCP options as sub-options. For example,
the IP Phone 2004 requires vendor specific Voice Gateway Media Card
information. The vendor, Nortel, decided to carry this information in one of
several Site Specific options and then encapsulate it into option 43. Since
Nortel Communication Server 1000
IP Phones Fundamentals
NN43001-368 05.02 26 May 2009
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