Proxy ARP
When a router answers ARP requests on behalf of another host — it gives its own MAC for an IP that actually lives in a different subnet. It lets hosts reach each other as if they were on the same network, without knowing about the router in between.
Use case (historical): a host with a wrong/missing netmask thinks the whole network is local and ARPs for an IP in another subnet; the router answers with its own MAC and forwards the traffic → it works "transparently" without correct routing config. Also used to bridge subnets or in certain WAN/VPN scenarios. Downsides: it hides the network topology, swells ARP tables, can mask misconfiguration, and be exploited in attacks. Modern view: avoid proxy ARP, configure routing properly instead — it's mostly a legacy artifact. Enabled/disabled per router interface. Distinct from gratuitous ARP (unsolicited updates) and ARP spoofing (malicious). Related to ARP and ARP spoofing.