Question : What does ARP function for?

Hi,

1. I am still confused with the function of "ARP" command
2. The thing that i knew that it related the "reserved IP address" to the "MAC address of a device" (such as, if the "reserved static IP is 162.16.2.1 and the MAC address of the device is, say it 00-60-B0-3B-31-69); if i use the command: arp -s 162.16.2.1 00-60-B0-3B-31-69, the device (say it a printer) will have the IP address of 162.16.2.1
3. So the function of arp is only making sure that the IP address is belong to a certain MAC address?
4. Could somebody elaborate more clearly the function of this ARP command (if possible pls give some examples)?
5. Thank you

Tjie

Answer : What does ARP function for?

ARP is a very useful function and can be used in many ways. As described above, one of these is to find a network device. If you think of the OSI table, MAC addresses operate at layer 2 ( A MAC address is the 12-digit ID that is burned into a network card chip to give it a unique identity). a tcpip address is a logical address that you manually assign with static ip addresses or dynamically through the DHCP service.

Lets say you have a subnet of 100 machines from 192,.168.1.10 - 192.168.1.110 with a 255.255.255.0 mask.
When 192.168.1.10 wants to talk to 192.168.1.100 for example, 192.168.1.10 puts out a broadcast to 192.168.1.255 - ie ALL machines on the 192,.168.1.0 subnet. ALL machines will see the broadcast arrive and ALL machines (except 192.168.1.100) will read the first few bytes and realise the broadcast does not concern them as they are NOT 192.168.1.100. These machines then ignore the rest of the broadcast. However, 192.168.1.100 realises that it is the intended recipient and so it replies, putting its MAC address into the packet back to 192.168.1.10. Now 192.168.1.10 can start a conversation with 192.168.1.100 directly as it knows the MAC address of 192.168.1.100.

A second use of ARP is when you use multiple IP addresses on a single nic. A good example of this is on a firewall or router where you have been given a number of external IP addresses by your ISP. You assign these ip's to the external interface but all of them will return the same MAC address to the calling machine.

A third use is for troubleshooting. You can use the arp command manually from the command line with the paramters provided to identify a beaconing work station for example.

Keith