Counts IP datagrams discarded because no route could be found to their destination, including forwarded packets that hit this condition, and specifically covers the case where all of a host's default gateways are down. A rising ipOutNoRoutes on a firewall is a strong, specific signal of an upstream routing failure, most notably all configured default gateways being unreachable, which is a very different and more urgent fix than a simple policy block. Example: ipOutNoRoutes suddenly climbing across every gateway in a cluster at the same time, with no policy change logged, points straight at an ISP or upstream router outage rather than a firewall misconfiguration.