∙file: Bootfile name and path information, defined by the server to the client.

∙options: Optional parameters field that is variable in length, which includes the message type, lease, domain name server IP address, and WINS IP address.

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.

a shows the DHCP option format.

a.DHCP option format

0

7

15

 

Option type

Option length

 

 

 

 

Value (variable)

 

 

 

Introduction to DHCP options

The common DHCP options are as follows:

∙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 67: Bootfile name option. It specifies the bootfile name to be assigned to the client.

∙Option 150: TFTP server IP address option. It specifies the TFTP server IP address to be assigned to the client.

∙Option 121: Classless route option. It specifies a list of classless static routes (the destination addresses in these static routes are classless) that the requesting client should add to its routing table.

∙Option 33: Static route option. It specifies a list of classful static routes (the destination addresses in these static routes are classful) that a client should add to its routing table. If Option 121 exists, Option 33 is ignored.

For more information about DHCP options, see RFC 2132.

Introduction to Option 82

Some options, such as Option 82, have no unified definitions in RFC 2132.

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