794 CHAPTER 56: DHCP OVERVIEW
server sent a reply back by broadcast. The remaining bits of the flags field are
reserved for future use.
■ ciaddr: Client IP address.
■ yiaddr: ’your’ (client) IP address, assigned by the server.
■ siaddr: Server IP address, from which the clients obtained configuration
parameters.
■ giaddr: The first relay agent IP address a request message traveled.
■ chaddr: Client hardware address.
■ sname: The server host name, from which the client obtained configuration
parameters.
■ file: Bootfile name and routing information, defined by the server to the client.
■ options: Optional parameters field that is variable in length, which includes the
message type, lease, DNS IP address, WINS IP address and so forth.
DHCP Options
DHCP Options Overview The DHCP message adopts the same format as the Bootstrap Protocol (BOOTP)
message for compatibility, but differs from it in the option field, which identifies
new features for DHCP.
DHCP uses the option field in DHCP messages to carry control information and
network configuration parameters, implementing dynamic address allocation and
providing more network configuration information for clients.
Figure 242 shows the DHCP option format.
Figure 242 DHCP option format
Introduction to DHCP
Options
The common DHCP options are:
■ Option 6: DNS server option. It specifies the DNS server IP address to be
assigned to the client.
■ Option 51: IP address lease option.
■ Option 53: DHCP message type option. It identifies the type of the DHCP
message.
■ Option 55: Parameter request list option. It is used by a DHCP client to request
specified configuration parameters. The option contains values that correspond
to the parameters requested by the client.
■ Option 66: TFTP server name option. It specifies a TFTP server to be assigned to
the client.
Option type Option length
07 15
Value (variable)
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