When a packet is created on a device, the IP protocol determines if the destination IP is on the local or remote network. In this scenario the destination IP (PC4) is in another network, therefore the IP packet must travel through its default gateway which in this case is R1. PC1 encapsulates the IP packet into a frame with R1s MAC address as the destination MAC address. Once the frame is received, R1 proceeds to discard the frame and re-encapsulate the packet with new R2 MAC address information. The router must route the packet through its S0/0 interface to R2. Once R2 receives the frame from S0/0 interface, it proceeds to do the same, discard the frame coming from R1 and checking the IP packet. Since that IP destination (PC4) is known by R2, it will proceed to encapsulate the packet with destination hardware address of PC4, and source hardware address of R2 F0/0 interface. Therefore the source physical address (MAC) that is shown in the frame of the IP packet sent from PC1 to PC4 is that of R2s F0/0.