If this query is sent to C.ISI.EDU, the reply would be:
+---------------------------------------------------+ Header | OPCODE=SQUERY, RESPONSE | +---------------------------------------------------+ Question | QNAME=BRL.MIL, QCLASS=IN, QTYPE=A | +---------------------------------------------------+ Answer | <empty> | +---------------------------------------------------+ Authority | MIL. 86400 IN NS SRI-NIC.ARPA. | | 86400 NS A.ISI.EDU. | +---------------------------------------------------+ Additional | A.ISI.EDU. A 26.3.0.103 | | SRI-NIC.ARPA. A 26.0.0.73 | | A 10.0.0.51 | +---------------------------------------------------+
This response has an empty answer section, but is not authoritative, so it is a referral. The name server on C.ISI.EDU, realizing that it is not authoritative for the MIL domain, has referred the requester to servers on A.ISI.EDU and SRI-NIC.ARPA, which it knows are authoritative for the MIL domain.