ARP

The Address Resolution Protocol (ARP) is a network protocol which allows mapping network addresses to hardware addresses.

To map an IP address to a MAC address, an ARP request is used. After encapsulating this request in a frame, it is sent out as a broadcast. All stations in the network receive and evaluate this request. They compare the IP address submitted in the ARP request with their own IP address. The station whose the IP address matches the one in the request replies by sending an ARP response.

The response is sent specifically to the station which had sent out the request, it contains the requested MAC address.

The MAC address is added to an ARP cache along with the associated IP address. The ARP cache is a temporary memory which is part of the respective station's RAM. Each time another IP address needs to be resolved to a MAC address, the station first checks whether it can do so using the ARP cache and thus without sending a new ARP request. If this is not possible, it sends an ARP request. Thus, ARP resolution is advantageous as it saves time and reduces the network load.

If an entry in the ARP cache is not accessed for a certain period of time, it will be removed. This behavior ensures that changes in the network can be detected and reflected by sending new ARP requests.

See also: ARP Cache Stati