If you are in the network 192.168.1.0/24 and try to ping 192.168.1.50 the client sends an ARP request. But if we try to ping 192.168.3.10 the client gets so smart, that it is not trying to send a ARP request (Broadcast) for that IP address, because it realized “that guy is not in my network”. So, it sends the request to the router (Default Gateway) because the router does two things:
- Connect networks (with his routing table)
- Stops broadcasts
So if you only could put one address (the physical address instead of the ip address) into your request, that would not work. So, two layers of addressing means I can put two addresses into my request.
So in my request I can put now a layer 2 address (mac address) of my router f.e. 4444.4444.4444 and into the “real” destination address I can put 192.168.3.10. So now we know, we have to go to the default gateway and still have the destination address available.