Broadcast Arp request who-has

What is ARP (Address Resolution Protocol)?

In addition to the standard function presented so far, there are a number of alternative uses for ARP message packets. For example, a host can also use this to inform the other network participants about its own MAC address. For this purpose it sends an ARP request to the broadcast MAC address and carries the own IP both as source and recipient address a. Since this happens of its own accord, this modification of the protocol is also called Gratuitous ARP (Eng. "Unsolicited ARP"). Such a self-initiated notification of the physical address can e.g. B. happened for the following reasons:

  1. Because of their IP addressing, GARP messages should in theory not trigger any response - otherwise this would suggest that the IP address used by the sender would be assigned twice. If one of the hosts contacted by broadcast nevertheless responds with a response message, this is a for the network administrator Indication of a faulty configuration.
  2. Gratuitous ARP enables all hosts to include the hardware address of the sender in their own cache. This is particularly advantageous if this address is different because of a Exchange of the network card has changed. Usually, the modified ARP messages are therefore always sent when a computer boots.
  3. Servers that are configured as servers and backup servers for the purpose of failure safety often share an IP address. So that the data traffic in an emergency redirected to the incoming server it must announce its MAC address to the other hosts. GARP messages are an excellent option for this.
  4. Mobile IP enables a mobile host to keep its own IP address even when changing networks. For this purpose, a so-called home agent takes over the IP of the mobile host when it leaves the home network. To draw attention to himself, he sends a gratuitous ARP message. The mobile host reports back in the same way when it is back in the home network.

Another variation is that Reverse Address Resolution Protocol (RARP) which basically reverses the function of ARP: It does not resolve IP addresses into MAC addresses, but MAC addresses into IP addresses. From this possibility that determine your own internet address, make use of devices that are not designed for assignment or permanent storage (e.g. workstations without a hard disk drive).

The modification is also widespread Proxy ARP, which enables routers to answer ARP requests for other hosts. This enables communication between two hosts from different subnets without having to change the network configurations of the devices. If an ARP request reaches a router with activated proxy ARP, the router responds instead of the actual target computer. It transmits its own MAC address, whereupon it then also the sender's data packets receives. The router then forwards the data to the destination host using the information from the ARP cache.