Routers will forward packets using a default route when there are no specific routes that match a packets destination IP address in the routing table. Routers process the default destination differently based on if ip classess is configured or not.
With ip classless configured the router automatically uses the default route if no specific route exists.
Without ip classes configured the router first checks to see if any part of the destination address’s classful network is in the routing table, before using the default route.
Default routes can be advertised using 4 methods. depending on routing protocol: