ARP (Address Resolution Protocol) is used for locating IPv4 systems on a local network by MAC address.
The ARP table in pfSense displays a list of systems on the network that have attempted to talk to or through the pfSense firewall within the past few minutes. If a system is up but has not talked to (or through) the pfSense firewall it will not show up in the ARP table.
To view the list of systems currently seen by pfSense, click Diagnostics > ARP Table. This list shows the IP Address, MAC Address, Hostname and the Interface where each system was last seen.
For IPv6 hosts, see NDP Table.