|  | 11.3 | Advanced ARP Concepts
| 11.3.4 | Proxy ARP

Proxy ARP is a variation of the ARP protocol. In this case an intermediate device (e.g. router) sends an ARP response, on behalf of an end node, to the requesting host. Routers running proxy ARP capture ARP packets. They respond with their MAC addresses for those requests in which the IP address is not in the range of addresses of the local subnet.

In the previous description of how data is sent to a host on a different subnet, the default gateway is configured. If the source host does not have a default gateway configured, it sends an ARP request. All hosts on the segment, including the router, receive the ARP request. The router compares the IP destination address with the IP subnet address to determine if the destination IP address is on the same subnet as the source host.

If the subnet address is the same, the router discards the packet. The reason that the packet is discarded is that the destination IP address is on the same segment as the source's IP address. This means another device on the segment should respond to the ARP request. The exception to this is that the destination IP address is not currently assigned, which will generate an error response on the source host.

If the subnet address is different, the router will respond with its own MAC address for the interface that is directly connected to the segment on which the source host is located. This is the proxy ARP. Since the MAC address is unavailable for the destination host, the router supplies its MAC address in order to get the packet. Then the router can forward the ARP request (based on the destination IP address) to the proper subnet for delivery.