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Address Resolution Protocol or ARP is used on a network by a transmitting host to find the MAC address of a destination with a known IP address. This is achieved by sending an ARP broadcast frame across the network; every host on the network segment will analyze this frame, and the host with the requested IP address will respond with its MAC address. The transmitting host then adds this information to its ARP table for future reference. ARP tables are also added to by analyzing network traffic and gathering MAC and IP address pairs.

The reverse is performed by Reverse Address Resolution Protocol which finds an IP address from a known MAC address.

MAC address <--ARP-- IP address
MAC address --RARP-> IP address

The command

The command arp can be used to display ARP information to the user:

# arp
Address                  HWtype  HWaddress           Flags Mask            Iface              ether   00:1d:7e:b4:e9:30   C                     eth1