This read-only Counter reports the number of times this node sent an ICMP destination-unreachable message because a requester tried to send a datagram to a network this node couldn't reach. An admin monitoring this node's role as a router would watch this counter to see how often it's rejecting traffic aimed at unreachable networks on behalf of others. For example, a spike in cnticmpOutDuNets would indicate clients behind this node are attempting to reach a network that has become unreachable.