Layer 2 needs to encapsulate the Layer 3 packet into a Layer 2 frame. To do this, Layer 2 needs to map the Layer 3 destination address of the packet to its MAC address. It does this by requesting a mapping from the ARP program.

ARP checks its table. In this example, it is assumed that this host has not communicated with the other host, so you see no entry in the ARP table. This results in Layer 2 holding the packet until ARP can provide a mapping. Figure 1-65 shows this operation.

Figure 1-65 ARP Table Lookup

Layer 2: ARP, do you have a mapping for

ARP: Is in my ARP table? No, Layer 2 will have to put the packet in the parking lot until I do an ARP.

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