Address Resolution Protocol (ARP)
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ARP is a protocol that maps an IP address to the hardware address (MAC) of a network interface.
The arp command is used to view or modify the kernel’s ARP table. Using arp, you can detect problems such as duplicate addresses on the network. Alternatively, you can use arp to manually add the required entries from your LAN.
“Remember that each IP packet must be encapsulated in a Layer 2 frame before it can be sent to the next hop. The Address Resolution Protocol (ARP) is used to determine the destination media access control (MAC) address for that frame.”
“After a client has resolved a name to an IP address, it determines whether that IP address is on its local subnet. If it is, it issues an ARP broadcast on the segment asking for the holder of that IP address to respond with its MAC address. If the destination host is not on the local subnet, the client issues an ARP request for the default gateway IP address.”
“ARP requests are not issued for every bit of communication. As a client makes requests, the replies are remembered in the ARP cache. Each time a client needs to encapsulate a packet in a frame, it checks the ARP cache to see if it has a MAC match for the destination IP.”