are sent between IP addresses, but the
in which they are packaged
are sent between MAC (hardware) addresses. Therefore the
MAC address of the destination IP address must be known before an IP
packet can be sent onto an Ethernet network.
The Address Resolution Protocol
(ARP) is used to obtain MAC address information. FreeRTOS+TCP
(like most if not all IP stacks) stores IP address to MAC address mappings
in an ARP table (sometimes called the ARP cache). ARP is handled
automatically by the TCP/IP stack.