The TCP/IP Guide - Summary Of DHCP Options / BOOTP Vendor Information Fields

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Last but certainly not least, Table 198 describes the DHCP-only options that control the operation of the DHCP protocol.

**Table 198: DHCP Options: DHCP Extensions**
Code Value	Data Length (bytes)	Name and Description
50	4	*Requested IP Address:* Used in a client's DHCPDISCOVER message to request a particular IP address assignment.
51	4	*IP Address Lease Time:* Used in a client request to ask a server for a particular DHCP lease duration, or in a server reply to tell the client the offered lease time. It is specified in units of seconds.
52	1
53	1
54	4	*Server Identifier:* The IP address of a particular DHCP server. This option is included in messages sent by DHCP servers to identify themselves as the source of the message. It is also used by a client in a DHCPREQUEST message to specify which server’s lease it is accepting.
55	Variable	*Parameter Request List:* Used by a DHCP client to request a list of particular configuration parameter values from a DHCP server.
56	Variable	*Message:* Used by a server or client to indicate an error or other message.
57	2	*Maximum DHCP Message Size:* Used by a DHCP client or server to specify the maximum size of DHCP message it is willing to accept. The minimum legal value is 576 bytes.
58	4	*Renewal (T1) Time Value:* Tells the client the value to use for its renewal timer.
59	4	*Rebinding (T2) Time Value:* Tells the client what value to use for its rebinding timer.
60	Variable	*Vendor Class Identifier:* Included in a message sent by a DHCP client to specify its vendor and configuration. This may be used to prompt a server to send the correct vendor-specific information using option #43.
61	Variable	*Client Identifier:* Used optionally by a client to specify a unique client identification for itself that differs from the DHCP default. This identifier is expected by servers to be unique amongst all DHCP clients and is used to index the DHCP server's configuration parameter database.
66	Variable	*TFTP Server Name:* When the DHCP message's SName field has been used for options using the option overload feature, this option may be included to specify the TFTP server name that would normally appear in the SName field.
67	Variable	*Bootfile Name:* When the DHCP message's File field has been used for options using the option overload feature, this option may be included to specify the bootfile name that would normally appear in the File field.