Any one knows what id's full meaning?

[Larry Liu]

Below is a nslookup result with "set debug".  Does anyone have a full 
understanding  for id=15 in line 3 ?  Is the the 15th query from a 
particular client before the ttl, expiration day, or otherwise?  Thanks.

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Got answer:
    HEADER:
        opcode = QUERY, id = 15, rcode = NOERROR
        header flags:  response, want recursion, recursion avail.
        questions = 1,  answers = 0,  authority records = 1,  additional = 0

    QUESTIONS:
        cnf.com, type = A, class = IN
    AUTHORITY RECORDS:
    ->  cnf.com
        ttl = 7200 (2 hours)
        primary name server = dmrqs004.cnf.com
        responsible mail addr = dns_id.dmrqs004.cnf.com
        serial  = 2003110400
        refresh = 3600 (1 hour)
        retry   = 1800 (30 mins)
        expire  = 604800 (7 days)
        default TTL = 7200 (2 hours)

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Name:    cnf.com