Once the router with a known route to a host destination is reached, the router determines which route is valid by finding the "most specific match". The network with the longest subnet mask that matches the destination IP address wins.
The default route in IPv4 (in CIDR notation) is 0.0.0.0/0, often called the quad-zero route. 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. Similarly, in IPv6 the default address is given by ::/0.
Routers in an organization generally point the default route towards the router that has a connection to a network service provider. This way, packets with destinations outside the organization's local area network (LAN)—typically to the Internet, WAN, or VPN—will be forwarded by the router with the connection to that provider.
Once it is routed outside the network, if that router does not know the route of the destination, it will forward it to its own Default Route, which is usually a router connected to larger number of networks. Similarly, the packet will progress to internet back bone if still no route is known about the destination IP. It is then considered that the network does not exist, and the packet is discarded.
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