This counter counts packets dropped because they weren't IPv4, with the MIB explicitly noting IPv6 packets are not supported by the router. This is a hard compatibility limit rather than a transient fault -- any device or upstream network element that starts sending IPv6 traffic toward this CDD-564 will have every one of those packets silently dropped and counted here, a common gotcha when integrating this legacy router into a dual-stack network. If a site upgrades its LAN equipment and starts seeing this counter climb for the first time, that's a strong signal a newly added device or DHCPv6-enabled host has started emitting IPv6 traffic that this router simply cannot forward.