Apart from the previously introduced standard function, there are a number of alternative uses for ARP message packages. For example, a host can also use it to inform the other network members about their own MAC address. For this purpose, it sends an ARP request to the broadcast and enters its own IP as both a source address and a recipient address. Since this is done on its own, this modification of the protocol is also called Gratuitous ARP (“Unsolicited ARP”). Such a self-initiated message of the physical address may occur, for example, in the following cases:
- Due to their IP addressing, GARP messages are not allowed to produce any response in the theory – otherwise, it would indicate that the IP address used by the sender would be duplicated. If one of the broadcast-contacted hosts replies with a response message anyway, is it the fault of the network administrator
- Gratuitous ARP allows all hosts to include the hardware address of the sender in their own cache. This is particularly advantageous when this address has changed, for example, because of the network card being exchanged. Therefore, the modified ARP messages are always sent when a computer boots up
- Servers that are configured as spares for the purpose of fail-protection often share an IP address. In order for data traffic to be redirected to the incoming server, the latter must announce the MAC address to the other hosts. For this, GARP messages are an excellent option
- Mobile IP allows a mobile host to keep its own IP address even when a network is switched. To this end, a so-called HomeAgent takes over the IP address of the mobile hosts when it’s removed from the home network. In order to draw attention to themselves, they send a gratuitous ARP message. In the same way, the mobile host logs back when it is back on the home network
A further modification is the Reverse Address Resolution Protocol (RARP), which basically inverts the function of the ARP: it does not resolve IP addresses in MAC addresses, but rather MAC addresses in IP addresses. This is the possibility to determine your own internet address by using devices in which the allocation or permanent storage is not provided (e.g., work stations without a hard disk drive).
The modification proxy ARP is also common, which enables routers to answer ARP requests for other hosts. This allows communication between two hosts from different subnets without the need for any changes to the devices’ network configurations. If an ARP request reaches a router with ARP proxy enabled, it responds instead of the actual target computer. It transmits its own MAC address, whereupon it also receives sender’s data packets. Subsequently, the router forwards the data to the target host, using the ARP cache information.