Address Resolution Protocol

The ARP command accesses the ARP cache which stores information gained by the ARP Address Resolution Protocol. When a computer wants to send a IP message to another computer in its subnet, it must first determine the hardware or Ethernet address. ARP requests are broadcasts that contain the destination IP address. The destination computer will recognize its IP address in the ARP request and then it will return its IP address in an ARP reply message to the originator.

èARP request = I know your IP address, but what is your Ethernet address?è çARP reply = Here is my Ethernet addressç

The ARP cache is a memory resident table of IP addresses and the corresponding Ethernet addresses. The ARP command can display and modify the ARP cache. The ARP cache is consulted when a computer wants to access an IP address. If the ARP cache does not contain the desired IP address, then the computer will send an ARP request broadcast. The ARP reply provides the corresponding Ethernet address which dynamically updates the ARP cache. Dynamic entries are removed after a period of time. Permanent or static entries can be put in the ARP cache with the ARP command.

USAGE
Displays and modifies the IP-to-Physical address translation tables used by
address resolution protocol (ARP).
ARP -s inet_addr eth_addr [if_addr]
ARP -d inet_addr [if_addr]
ARP -a [inet_addr] [-N if_addr]
-a Displays current ARP entries by interrogating the current
protocol data. If inet_addr is specified, the IP and Physical
addresses for only the specified computer are displayed. If
more than one network interface uses ARP, entries for each ARP
table are displayed.
-g Same as -a.
inet_addr Specifies an internet address.
-N if_addr Displays the ARP entries for the network interface specified
by if_addr.
-d Deletes the host specified by inet_addr. inet_addr may be
wildcarded with * to delete all hosts.
-s Adds the host and associates the Internet address inet_addr
with the Physical address eth_addr. The Physical address is
given as 6 hexadecimal bytes separated by hyphens. The entry
is permanent.
eth_addr Specifies a physical address.
if_addr If present, this specifies the Internet address of the
interface whose address translation table should be modified.
If not present, the first applicable interface will be used.
Example:
> arp -s 157.55.85.212 00-aa-00-62-c6-09 .... Adds a static entry.
> arp -a .... Displays the arp table.

C:\>arp -a
Interface: 0.0.0.0 --- 0xffffffff
Internet Address Physical Address Type
209.47.184.40 00-03-47-12-9d-49 dynamic
209.47.184.41 00-10-a4-a5-99-3c dynamic
209.47.184.42 00-e0-29-44-03-21 dynamic
209.47.184.43 00-10-5a-a7-33-03 dynamic
209.47.184.45 00-00-f0-a0-5d-40 dynamic