ARP is the Address Resolution Protocol as described in RFC 826. ARP is used by a networked machine to resolve the hardware location/address of another machine on the same local network. Machines on the Internet are generally known by their names which resolve to IP addresses. This is how a machine on the foo.com network is able to communicate with another machine which is on the bar.net network. An IP address, though, cannot tell you the physical location of a machine. This is where ARP comes into the picture.
Let's take a very simple example. Suppose I have a network composed of several machines. Two of the machines which are currently on my network are foo with an IP address of 10.0.0.1 and bar with an IP address of 10.0.0.2. Now foo wants to ping bar to see that he is alive, but alas, foo has no idea where bar is. So when foo decides to ping bar he will need to send out an ARP request. This ARP request is akin to foo shouting out on the network "Bar (10.0.0.2)! Where are you?" As a result of this every machine on the network will hear foo shouting, but only bar (10.0.0.2) will respond. Bar will then send an ARP reply directly back to foo which is akin bar saying, "Foo (10.0.0.1) I am here at 00:60:94:E9:08:12." After this simple transaction that's used to locate his friend on the network, foo is able to communicate with bar until he (his arp cache) forgets where bar is (typically after 15 minutes on Unix).
Now let's see how this works. You can view your machines current arp/neighbor cache/table like so:
[root@espa041 /home/src/iputils]# ip neigh show 220.127.116.11 dev eth0 lladdr 00:60:08:3f:e9:f9 nud reachable 18.104.22.168 dev eth0 lladdr 00:06:29:21:73:c8 nud reachable
As you can see my machine espa041 (22.214.171.124) knows where to find espa042 (126.96.36.199) and espagate (188.8.131.52). Now let's add another machine to the arp cache.
[root@espa041 /home/paulsch/.gnome-desktop]# ping -c 1 espa043 PING espa043.austin.ibm.com (184.108.40.206) from 220.127.116.11 : 56(84) bytes of data. 64 bytes from 18.104.22.168: icmp_seq=0 ttl=255 time=0.9 ms --- espa043.austin.ibm.com ping statistics --- 1 packets transmitted, 1 packets received, 0% packet loss round-trip min/avg/max = 0.9/0.9/0.9 ms [root@espa041 /home/src/iputils]# ip neigh show 22.214.171.124 dev eth0 lladdr 00:06:29:21:80:20 nud reachable 126.96.36.199 dev eth0 lladdr 00:60:08:3f:e9:f9 nud reachable 188.8.131.52 dev eth0 lladdr 00:06:29:21:73:c8 nud reachable
As a result of espa041 trying to contact espa043, espa043's hardware address/location has now been added to the arp/neighbor cache. So until the entry for espa043 times out (as a result of no communication between the two) espa041 knows where to find espa043 and has no need to send an ARP request.
Now let's delete espa043 from our arp cache:
[root@espa041 /home/src/iputils]# ip neigh delete 184.108.40.206 dev eth0 [root@espa041 /home/src/iputils]# ip neigh show 220.127.116.11 dev eth0 nud failed 18.104.22.168 dev eth0 lladdr 00:60:08:3f:e9:f9 nud reachable 22.214.171.124 dev eth0 lladdr 00:06:29:21:73:c8 nud stale
Now espa041 has again forgotten where to find espa043 and will need to send another ARP request the next time he needs to communicate with espa043. You can also see from the above output that espagate (126.96.36.199) has been changed to the "stale" state. This means that the location shown is still valid, but it will have to be confirmed at the first transaction to that machine.