Address Resolution Protocol

Address Resolution Protocol (ARP) is used to resolve IP addresses to MAC addresses in an Ethernet network. A host wanting to obtain a physical address broadcasts an ARP request onto the TCP/IP network. The host on the network that has the IP address in the request then replies with its physical hardware address. When a MAC address is determined, the IP address association is stored in an ARP cache for rapid retrieval. Then the IP datagram is encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet is specified by the Subnetwork Access Protocol (SNAP). Reverse Address Resolution Protocol (RARP) works the same way as ARP, except that the RARP request packet requests an IP address rather than a MAC address. Use of RARP requires a RARP server on the same network segment as the router interface. RARP often is used by diskless nodes that do not know their IP addresses when they boot. The Cisco IOS Software attempts to use RARP if it does not know the IP address of an interface at startup. Also, Cisco routers can act as RARP servers by responding to RARP requests that they can answer.