Why does the WiFi $(802\mathrm{.}11)$ link$-$layer frame have three addresses? $[$Note: WiFi actually has four MAC addresses in the frame, but we're only focusing here on the three widely used ones$].$ Because there may be two hosts or routers that are possible destinations for this link$-$layer frame and we need to identify which of these is the intended receiver. Because the sender of this frame can be either the access point or a link$-$layer host or router interface, and we need to identify which of these two is the sender$.$ Because both the access point that will relay this frame to the intended link$-$layer receiving host or router interface, as well as that intended destination host or router interface need to be specified.