When is a default route used within a routing table?

A default route is used in a routing table when a more specific match for a destination address cannot be found. In IP networking, routers maintain a routing table that includes specific routes to known networks. However, there may be instances where a destination network does not have a specific entry in the routing table.

In such situations, the default route acts as a catch-all pathway, allowing the router to forward packets destined for unknown networks to a designated next-hop address, instead of dropping them. This is particularly useful in simplifying routing configurations and ensuring that traffic can still be directed outside of the local routing environment.

For instance, if a router receives a packet for a destination that it does not have a specific route to, it can seamlessly forward that packet to the next-hop defined in the default route. This allows for more efficient and flexible routing, especially in large networks or in scenarios where dynamic routing protocols are utilized.

The other choices do not accurately describe the purpose of a default route: it’s not used when no routes exist at all, it doesn’t pertain to static routes exclusively, and it certainly isn't meant for unreachable routes, as a default route helps in managing traffic for reachable but unspecified destinations.