Counts input datagrams discarded because the destination IP address in their header wasn't valid for this device to receive, including invalid addresses like 0.0.0.0 and unsupported address classes. On a firewall this is a useful early signal of misrouted or spoofed traffic reaching an interface it shouldn't; a steady trickle is normal internet noise, but a sudden burst aimed at one interface suggests someone is probing with malformed destination addresses. Example: ipInAddrErrors spiking on the external interface right before a fwRejected spike suggests the same probing traffic is using bad destination addresses as part of its scan.