ARP Switches and an Example

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
-N	if_addr Displays the ARP entries for the network interface specified by if_addr.
-d	Deletes the host specified by inet_addr.
-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.

An example. This is from my machine at the present. I'm coonected to MSN through dial up and the first interface is the IP address of the 1Cust51.max44.Seattle.WA.ms.uu.net which is me presently.
The physical interface is the MAC - nope not Macintosh but Media Access Control which is the hardware address of the interface being used. The IP addresses are places I'm accessing or have been accessing lately.

C:\>arp -g

Interface: 153.34.131.179 on Interface 3
Internet Address           Physical Address           Type
16.1.0.4                        20-53-52-43-00-00       dynamic
128.173.14.71              20-53-52-43-00-00       dynamic
129.132.98.11              20-53-52-43-00-00       dynamic
192.31.216.8                20-53-52-43-00-00       dynamic
204.118.34.6                20-53-52-43-00-00       dynamic
204.118.34.22              20-53-52-43-00-00       dynamic
204.123.2.72                20-53-52-43-00-00       dynamic
204.255.246.18            20-53-52-43-00-00       dynamic
208.215.43.40              20-53-52-43-00-00       dynamic

Each network card has a 48 bit MAC address. The address space is assigned for each OEM such that from the card address (hardcoded into the card) you could determind the OEM. Try link 2 below

Want more info and become an expert try these links:

ARP	ZDNet Webopedia
Cavebear	Ethernet codes (trivia) - you can find the card OEM here if you read the address