Understanding Routers and ARP
Routers segment networks. Segmenting a network with routers reduces network traffic because, by default, routers do not forward broadcast traffic from one segment to another.
Since hosts rely on ARP requests to determine MAC addresses, and ARP requests use broadcasts, hosts must create broadcast traffic in order to determine the MAC address of other hosts.
Since routers filter broadcast traffic, ARP requests can go no farther than the local segment.
When a source host needs the MAC address of a host on a remote segment, the source must forward the packet to the router, aka the default gateway.