DHCP uses the BOOTP message format defined in RFC 951 and given in table 1 and figure 1. The 'op' field of each DHCP message sent from a client to a server contains BOOTREQUEST. BOOTREPLY is used in the 'op' field of each DHCP message sent from a server to a client.
The first four octets of the 'options' field of the DHCP message contain the (decimal) values 99, 130, 83 and 99, respectively (this is the same magic cookie as is defined in RFC 1497 ). The remainder of the 'options' field consists of a list of tagged parameters that are called "options". All of the "vendor extensions" listed in RFC 1497 are also DHCP options. RFC 1533 gives the complete set of options defined for use with DHCP.
Several options have been defined so far. One particular option - the "DHCP message type" option - must be included in every DHCP message. This option defines the "type" of the DHCP message. Additional options may be allowed, required, or not allowed, depending on the DHCP message type.
Throughout this document, DHCP messages that include a 'DHCP message type' option will be referred to by the type of the message; e.g., a DHCP message with 'DHCP message type' option type 1 will be referred to as a "DHCPDISCOVER" message.