Arp

Arp (Arp.exe) is a useful command-line tool for diagnosing problems in connecting to systems on a LAN where communications between computers do not travel through a router. Arp is also useful for diagnosing problems related to the client communicating with the default gateway. When a client contacts a server on the same subnet, it must address the frame with both the media access control (MAC) address and the IPv4 address. The MAC address is a 48-bit number that uniquely identifies a network adapter.

Arp is the name of a tool; it is also the acronym for the Address Resolution Protocol (ARP), which is used to find the MAC address corresponding to an IPv4 address. When a client communicates with a system on the same LAN, ARP broadcasts a message to all systems on the LAN asking for a response from the system that has the requested IPv4 address. That system responds to the broadcast by sending its MAC address, and ARP stores the MAC address in the ARP cache.

Note IPv4 addresses are used to identify computers on different networks. However, computers communicating across a LAN use MAC addresses to identify each other. ARP lets a computer look up a MAC address based on an IPv4 address so that two computers on the same LAN can communicate.

Problems with ARP occur only occasionally. For example, if a system changes its network adapter, clients might store the incorrect MAC address in the ARP cache. You can also manually place MAC addresses into the ARP cache, but if a manually added MAC address is incorrect, communications sent to that IPv4 address will not succeed.