4.1. Message Header

4.1. Message Header

The DNS message header's second full 16-bit word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see Section 4.1.1 of [RFC1035]). Some of these flag values were marked for future use, and most of these have since been allocated. Also, most of the RCODE values are now in use. The OPT pseudo-RR specified below contains extensions to the RCODE bit field as well as additional flag bits.