Why Device Need A Default Route

Suppose that device B wants to ping device A. Since we need to know the mac address of the device, it will send a ARP request.

If device B has a default route, it can find the address of device A hence can ping to A.

However, device A doesn't know how to talk to device B, and without a default route, it will not be able to send the response

Therefore, in device A, we will need to setup a default route as the Network Gateway

sudo ip route add default via <gateway_ip>