Linux provides two very useful tools for diagnosing network troubles: arp and ip neigh.
The arp command is a tool that allows you to display the IP-address-to-MAC-address mappings that a system has built so that it doesn’t have to fetch the same information repeatedly for systems it communicates with. In doing this, arp allows you to discover and display details about systems on your network.
The other is the arp command’s younger brother, ip neigh, which can also display and manipulate arp tables. In this post, we’ll take a look at how these commands work and what they can tell you.
Source:: Network World – Linux