Allied Telesis AT-WR4500 manual Proxy ARP

90	AT-WR4500 Series - IEEE 802.11abgh Outdoor Wireless Routers
	RouterOS v3 Configuration and User Guide

AB

198.168.0.20/24

Network A

192.168.0.0/24

198.168.0.30/24

198.168.0.1/25
ether1
ether2
198.168.0.129/25
Network B		198.168.0.130/25
192.168.0.128/25
	C

Figure 11: Proxy ARP

Suppose the host A needs to communicate to host C. To do this, it needs to know host's C MAC address. As shown on the diagram above, host A has /24 network mask. That makes host A to believe that it is directly connected to the whole 192.168.0.0/24 network. When a computer needs to communicate to another one on a directly connected network, it sends a broadcast ARP request. Therefore host A sends a broadcast ARP request for the host C MAC address.

Broadcast ARP requests are sent to the broadcast MAC address FF:FF:FF:FF:FF:FF. Since the ARP request is a broadcast, it will reach all hosts in the network A, including the router R1, but it will not reach host C, because routers do not forward broadcasts by default. A router with enabled proxy ARP knows that the host C is on another subnet and will reply with its own MAC adress. The router with enabled proxy ARP always answer with its own MAC address if it has a route to the destination.

This behaviour can be usefull, for example, if you want to assign dial-in (ppp, pppoe, pptp) clients IP addresses from the same address space as used on the connected LAN.