I'm studying for CompTIA's Network+ exam and have gotten two different answers to this question. I know TCP/UDP operates on layer 4 (transport) and IP/ICMP on layer 3 (network), but where does ARP fall into the OSI scheme? Since it resolves IPs to MAC addresses I would have thought layer 2 (data link), as this page seems to indicate: http://www.mindflip.com/inet/sevenlayer.html
Yet other sources indicate layer 3: http://www.skillsoft.com/corporate/c...164/64164a.htm
So which is it? Looks like CompTIA wants the layer 3 answer, but they've been wrong before... like the A+ tests seem to think "ipconfig /all" only works on NT/2000 and you have to use the GUI winipcfg utility with 98/ME... which is completely wrong. The CLI command works on all of those OSes (and the similarity to "ifconfig -a" makes it easy to remember too.) So I'd like to know the right answer in addition to CompTIA's.

Thanks in advance.

Both ICMP and ARP (As will as IGMP and RARP) are somewhat controversial.

Some people will put ICMP on L3, while some other will put it on L4 (because in the IP packet header, there is a place for a Protocol ID, for TCP it's 6, for UDP it's 17, and for ICMP it's 1, which puts ICMP right there with TCP and UDP as a layer 4 protocol).

The same goes for ARP, some will say it is a L2 protocol (because it is used to resolve L2 addresses), while some others will say it is a layer 3 protocol (because IP is a L3 address, and a L2 device implementing L2 protocols only will not know that it has a IP address, and wouldn't allow you to configure one because it doesn't know you are talking about, so you'll need the L3 functionality).

If you are setting a CompTIA exam, and CompTIA wants ARP to be L3, then this is the answer you'll need, as you might get your answer marked as wrong if you answered otherwise.

Good luck !

Thanks for the reply jalal. Cisco's docs gave me a similar answer. They don't put it squarely on either layer, placing it between L2 and L3 instead: http://www.cisco.com/univercd/cc/td/...ito_doc/ip.htm
I guess that means L2.5 So I'll just round up for CompTIA.