proxy ARP

Proxy ARP is a technique for using the ARP protocol to provide an ad hoc routing mechanism.A multi-port networking device (e.g. a router) implementing Proxy ARP will respond to ARP requests on one interface as being responsible for addresses of device addresses on another interface. The device can then receive and forward packets addressed to the other devices.
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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. Subnetting would normally be a better solution.