...
ARP is only used when the IPv4 stack is enabled.
ARP caches the mapping of IP addresses to physical (i.e., MAC) addresses. NET_ARP_CFG_NBR_CACHE configures the number of ARP cache entries. Each cache entry requires approximately 18 bytes of RAM, plus five pointers, plus a hardware address and protocol address (10 bytes assuming Ethernet interfaces and IPv4 addresses).
The number of ARP caches required by the application depends on how many different hosts are expected to communicate. If the application only communicates with hosts on remote networks via the local network’s default gateway (i.e., router), then only a single ARP cache needs to be configured.
To test µC/TCP-IP with a smaller network, a default number of 3 ARP caches should be sufficient.
Neighbor Discovery Protocol (NDP) Configuration
...