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.

The advantage of Proxy ARP over other networking schemes is simplicity. A network can be extended using this technique without the knowledge of the upstream router.

For example, suppose a host, say A, wants to contact another host, say B, on an other subnet. For this, host A will send an ARP request with IP address of B in its ARP packet. The multihomed router which is connected to both the subnets, answers to host A's request with its MAC address instead of host B's actual MAC address thus proxying for host B. In the due course of time, when host A send a packet to the router which is actually destined to host B, the router just forwards the packet to host B. The communication between host A and B is totally unaware of the router proxying for each other.

Disadvantages include scalability (ARP resolution is required for every device routed in this manner) and reliability (no fallback mechanism is present, and the masquerading can be confusing in some environments).