By comparison to the total number of publicly accessible hosts on the Internet there is an almost insignificant number of hosts inside any locally reachable network. This means that the majority of potential destinations are only available via a router.
Any machine which will accept and forward packets between two networks is a router. Every router is at least dual-homed; one NIC connects to one network, and a second NIC connects to another network. Machines connected to either network learn by a routing protocol or are statically configured to pass traffic for the other network to the router.
For tristan, there are two different paths out of 192.168.99.0/24. One path has another leaf network, 192.168.98.0/24, and the other path has many networks, including the Internet. The routing table on tristan should then contain two different routes out of the network. One destination 192.168.98.0/24 will be reachable through 192.168.99.1. So, if tristan has a packet with a destination IP address in the range of the branch office network, it will choose to send the packet directly to isdn-router.
The default route is another way to say the route for destination 0/0. This is the most general possible route. It is the catch-all route. If no more specific route exists in a routing table, a default route will be used. Many servers and workstations are connected to leaf networks with only one router, hence Example 4.3 shows a very common sort of routing table. There's a route for localhost, for the locally connected IP network, and a default route.
For Internet-connected hosts, the default route is customarily set to the IP of the locally reachable router which has a path to the Internet. Each router in turn has a default gateway pointing to another Internet-connected router until the packet is handed off to an Internet Service Provider's network.