ARP Commands 89141ARP CommandsWhen a host has an IP packet to send on an Ethernet network, it mustencapsulate the IP packet in an Ethernet frame. The Ethernet header requiresa destination MAC address. If the destination IP address is on the samenetwork as the sender, the sender uses the Address Resolution Protocol (ARP)to determine the MAC address associated with destination IP address. Thenetwork device broadcasts an ARP request, identifying the IP address forwhich it wants a corresponding MAC address. The IP address is called thetarget IP. If a device on the same physical network is configured with thetarget IP, it sends an ARP response giving its MAC address. This MAC addressis called the target MAC.If the destination IP address is not on the same network as the sender, thesender generally forwards the packet to a default gateway. The defaultgateway is a router that forwards the packet to its destination. The host maybe configured with a default gateway or may dynamically learn a defaultgateway.The router discovery protocol is one method that enables hosts to learn adefault gateway. If a host does not know a default gateway, it can learn thefirst hop to the destination through proxy ARP. Proxy ARP (RFC 1027) is atechnique used to make a machine physically located on one network appearto be logically part of a different physical network connected to the samerouter (may also be a firewall). Typically Proxy ARP hides a machine with apublic IP address on a private network behind a router and still allows themachine to appear to be on the public network. The router proxies ARPrequests and all network traffic to and from the hidden machine to make thisfiction possible.Proxy ARP is implemented by making a small change to a router's processingof ARP requests. Without proxy ARP, a router only responds to an ARPrequest if the target IP address is an address configured on the interface wherethe ARP request arrived. With proxy ARP, the router may also respond if ithas a route to the target IP address. The router only responds if all next hopson its route to the destination are through interfaces other than the interfacewhere the ARP request was received.
