Default route

A default route is the network route used by a router when no other known route works for a given IP packet's destination address. A router determines which route is valid to reach an IP address by finding the "most specific match". That is, the network with the longest subnet mask that matches the IP address, wins.

The default route in IPv4 (in CIDR notation) is 0.0.0.0/0. Since the subnet mask given is /0, it effectively specifies no network, and is the "shortest" match possible. A route lookup that doesn't match anything, will naturally fall back onto this route.

Hosts and routers in an organization generally point the default route towards the router that has a connection to a network service provider. This way, packets destined towards the internet will be sent towards the router with the internet connection.