proxy arp

from The Free On-line Dictionary of Computing (8 July 2008)

proxy ARP

   <networking> The technique in which one {host}, usually a
   {router}, answers {Address Resolution Protocol} (ARP) requests
   intended for another machine.  By "faking" its identity, the
   router accepts responsibility for routing {packets} to the
   "real" destination.  Proxy ARP allows a site to use a single
   {IP address} with two physical networks.  {Subnet}ting would
   normally be a better solution.

   (2007-09-03)