So maybe I misunderstood the article, but I would expect that (assuming my router is on 192.168.0.123) that ping 192.168.0.123 followed by arp -a 192.168

So maybe I misunderstood the article, but I would expect that (assuming my router is on 192.168.0.123) that ping 192.168.0.123 followed by arp -a 192.168.0.123 should tell me the MAC address of the router. But instead I get "No ARP entries found." - what's going on here?