Forouzan §9.2. The destination decides the question. Pick a destination, press Send, and watch which IP address is handed to ARP.
What to look at: the IP pair on the frame never changes. The MAC pair does — and when the destination is off this network, the MAC that comes back belongs to the router, not to the destination.
A wants to send to…
Do it
A is 10.0.2.7 / A2:34:45:11:92:F1 · the default router is 10.0.2.1 / E0:11:22:33:44:55
The forwarding decision
Is the destination on this network?
—
So the next node is
—
IP handed to ARP
—
MAC that comes back
—
A’s ARP cache
IP
MAC
empty
The frame that finally leaves A
What happened on the wire
—
Press Send. Watch the difference between the two destinations — it is the whole point of the slide before this one.