A proxy is something that works on behalf of another entity. A proxy ARP is not actually an ARP type but a technique instead. Here are a few examples:

• We can use a proxy when a machine with a public IP address is in a private network behind a firewall. In this case, a way to resolve the MAC address is by using a proxy ARP, which conceals the existence of the hidden host behind the firewall and makes it appear as if it is in front of the firewall. The firewall uses a proxy ARP to and from the hidden device to maintain the illusion that the machine is on the public side.
• A proxy ARP can be used in a LAN when a host in one subnetwork is separated by a proxy router. When an ARP broadcast is sent to a host on another subnetwork, the router responds with its own MAC address and acts as a proxy to the host on the other subnetwork, as shown here:

You can now understand that there are many different types of ARP messages and techniques that may be used on a LAN. ARP is an essential protocol but can be a vulnerable target. In the next section, let's take a look at some ARP attacks and some defense methods.