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6. If there is no entry in the ARP cache then ARP issues an ARP Request. An ARP request is broadcast requesting ownership of the IP address. If there is a response to the ARP Request known as an ARP Response then the IP process knows that that Host B resides on the local network and then can tell Layer 2 to the physical address. If there is no ARP response then the IP process passes the MAC address of the local default gateway.
7. Back to the ARP response: If there is an ARP reponse then a new entry is added to the ARP table. The ARP table is then reconsulted by IP and the entry is found
So, as you can see, based on the idea that layer three protocols use SAPs (Length/Type) so their requests are passed to the proper Layer 3 protocol, as well as the dependence of the IP process on ARP processing for accomplishing path determination prior to passing the data to Layer 2 places ARP squarely in Layer 3.
Note: The processing outlined here is well within the confines of the definition of the processing that occurs at Layer 3, "path determination". Moreover, this process enables Layer 3 to be independent of Layer 2. discussion on ARP processing see RFC 826. For an excellent discussion on ARP and IP interaction, and implications of direct and indirect routing see RFC 1180.
I do appologize for this being so long. I hope this helps.
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