The history of DHCP and DHCP options traces back to the Bootstrap Protocol (BOOTP). BOOTP was used by a host to configure itself dynamically during its booting procedure. A host could receive an IP address and a file from which to download a boot program from a server, along with the server’s address and the address of an Internet gateway.

Included in the BOOTP packet was a vendor information field, which could contain a number of tagged fields containing various types of information, such as the subnet mask, the BOOTP file size, and many other values. RFC 1497 describes the BOOTP Vendor Information Extensions. DHCP replaces BOOTP; BOOTP is not supported on the firewall.

These extensions eventually expanded with the use of DHCP and DHCP host configuration parameters, also known as options. Similar to vendor extensions, DHCP options are tagged data items that provide information to a DHCP client. The options are sent in a variable-length field at the end of a DHCP message. For example, the DHCP Message Type is option 53, and a value of 1 indicates the DHCPDISCOVER message. DHCP options are defined in RFC 2132, DHCP Options and BOOTP Vendor Extensions.

A DHCP client can negotiate with the server, limiting the server to send only those options that the client requests.