ARP is at Layer 2 or Layer 3?

In some books i read that ARP protocol is at Layer 2 of TCP/IP stack but in some other it is mentioned there that the ARP is at Layer 3 of TCP/IP stack , where it is actually? i am confused about it.
Can you explain with authenticated references please
baburkhanAsked:
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NopiusCommented:
http://www.geocities.com/SiliconValley/Vista/8672/network/arp.html#A2

The confusion may come from: http://en.wikipedia.org/wiki/Address_Resolution_Protocol
"ARP can be used to resolve MAC addresses to many different Layer 3 protocols addresses. ARP has also been adapted to resolve other kinds of Layer 2 addresses; for example, ATMARP is used to resolve ATM NSAP addresses in the Classical IP over ATM protocol."

So definitly ARP works over Layer 2, but it used to resolve Layer 3 or other Layer 2 addresses.
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hanckeCommented:
Here;s the best short explanation I have seen.
ARP returns the layer 2 address for a layer 3 address.

http://www.answers.com/topic/address-resolution-protocol
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rsivanandanCommented:
Both the assumptions are correct.

If you take OSI layer, arp works in layer 3

If you take the old tcp/ip layer (which has only 4 layers), it works in layer 2.

To explain, first the tcp/ip stack was defined (DoD) and when the communications evolved, the tcp/ip layer is further expanded into 7 layers and that is why the layer shift.

http://www.cisco.com/univercd/cc/td/doc/product/iaabu/centri4/user/scf4ap1.htm

You could use the above link to correlate between them.
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