ARP (Address Resolution Protocol) is used for mapping a network address (e.g. an IPv4 address) to a physical address like an Ethernet address (also named a MAC address). Systems keep an ARP look-up table where they store information about what IP addresses are associated with what MAC addresses. When trying to send a packet to an IP address, the system will first consult this table to see if it already knows the MAC address. (Command Prompt: arp -a)