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The NDP Table, found under the Diagnostics menu in the pfSense® webGUI, shows the IPv6 Neighbor Discovery Protocol list. This list contains all of the current IPv6 peers, and is roughly analogous to the ARP Table for IPv4.
The NDP table shows the peer’s IPv6 address, its MAC address, the Hostname (if known), and the interface upon which that peer IP address was last observed.
As with ARP, these are only the hosts that are actively talking to or through the firewall, and is not a perfect indicator of a host’s actual online status. If a host is in the table, it is likely online, but if it is not in the list, it does not necessarily mean that it is offline, it may simply not have attempted external communication recently.