Dynamic ARP mappings expire automatically after a couple minutes. You'll only have dynamic ARP mappings for devices your machine has sent unicast packets to recently. Do a ping-scan of your subnet (or maybe just ping the subnet-broadcast address or the all-hosts multicast address) and then rerun arp -a
.
Also note that I believe the arp
command has been deprecated on modern Linux. The replacement is the ip
command, such as ip neighbor show
or abbreviated ip n show
.