The Free On-line Dictionary of Computing (30 December 2018):

constant mapping

    A precursor to ARP used by some TCP software in
   which the destination Ethernet address is constructed from the
   top 24 bits of the source Ethernet address followed by the low
   24 bits of the (class A) destination Internet address.  For this
   scheme the top 24 bits of the Ethernet address must be the same on
   all hosts on the network.

   (2014-08-06)