In RFC 951, the format of BOOTP packet was legislated, but the vendor information was not legislated in this document, so the authors of this document had described that :
"If the 'vend' field is used, it is recommended that a 4 byte 'magic number' be the first item within 'vend'. This lets a server determine what kind of information it is seeing in this field. "
I think it meant that the format of vendor information wasn't fixed in RFC 951, and any vendor can legislate a new format of vendor information by itself. And the value in "magic cookie" can be set by any vendor.
But in RFC 2131, the format of DHCP packet was legislated, and the "magic cooke" was fixed to values 99, 130, 83 and 99, I think it meant that the format of option information in DHCP packet was fixed absolutely and any vendor can't legislate a new format by itself.
My question are as follow:
Since the format of option information in DHCP packet was fixed absolutely, why the network device needs "magic cookie" to identify the mode in which the succeeding data is to be interpreted ? I think the magic cookie is not useful in DHCP packet because the format of option information is fixed. In other words, there is only one format of option information forever.
As mr. Ralph Droms which was editor of the RFC 2132 and RFC 2131 and chairman of DHCP working group said this 220.127.116.11 was fixed in RFC 1048. The fix of the value was intentended to make interoperability easier I assume. So in RFC 951 the use was intended to give addional vendor extentions but was fixed in RFC 1048 and since it is there in this format.
As far as I know the 18.104.22.168 was defined almost 10 years before RFC 2131 in RFC 1048. DHCP message type is recognized by DHCP message type option.
EDIT: I did find the old thread about it which aswered my question when I was wondering before on the same question:
In my understanding, DHCP messages are formatted in an identical way to BOOTP messages. Without the specific magic cookie, it would be impossible to differentiate between a BOOTP and a DHCP message. In fact, the fixed magic cookie present in the BOOTP message means that this is really a DHCP message, and everything that follows the magic number is to be interpreted as DHCP options.
So the magic value of 99, 130, 83 and 99 is not really a vendor's magic to differentiate its own options. Rather, it is a distinguisher to know that this is a DHCP message, and the "vendor options" are really DHCP options.