5. arp

Remember when we lookup a MAC address with ARP, it first checks the locally stored ARP cache on our system, you can actually view this cache:

[email protected]:~$ arp
Address HWtype HWaddress Flags Mask Iface ether 00:12:24:fc:12:cc C eth0 ether 00:12:45:f2:84:64 C eth0

The ARP cache is actually empty when a machine boots up, it gets populated as packets are being sent to other hosts. If we send a packet to a destination that isn't in the ARP cache, the following happens:

  1. The source host creates the Ethernet frame with an ARP request packet

  2. The source host broadcasts this frame to the entire network

  3. If one of the hosts on the network knows the correct MAC address, it will send a reply packet and frame containing the MAC address

  4. The source host adds the IP to MAC address mapping to the ARP cache and then proceeds with sending the packet

You can also view your arp cache via the ip command:

$ ip neighbour show


Observe what happens to your ARP cache when you reboot your machine and then do something on the network.


What command can you use to view your ARP cache?