Address Resolution Protocol (ARP) finds the Hardware Address of a host from a known IP address. When a client has an IP datagram to send and it doesn't have the destination device's hardware address in the ARP cache, it uses ARP messages to find such information. After that, the client will store the ARP mapping in the ARP local table. ARP will request the information to the local network using a layer 2 broadcast (FFFF:FFFF:FFFF). In the question, PC1s default gateway is R1. In the beginning PC1 already knows its own IP, mask and default gateway through manual configuration or DHCP. However, PC1 doesnt know the MAC address of the default gateway, so it will use ARP request broadcast messages to find it. For more information refer to: [Todd Lammle, CCNA Cisco Certified Network Associate, Study Guide, Fifth Edition, Chapter 2, page 80]