, 8], Intermediate System to Intermediate System (IS-IS) [9], and Border Gateway Protocol (BGP) [10]. Sometimes routers do not have the complete picture of the whole network, such as the whole Internet. A router might not know which of its interfaces would bring the packet to the right direction. Therefore, the router may have a default route. The default route basically states that if there is no more specific information - a specific route that fits the packet's destination address - by default the packet should be sent through a specific interface to a specific router. Directly connected routers exchange information about the routes they have gotten from other routers to which they are connected. A router uses this information to create a view of the network topology. This network topology map is stored in an internal data structure called a routing table. When some new information about new routes or old routes disappearing comes to the router either by the routing protocol updates, or by the router noticing a neighboring router disappearing, the router updates the internal routing table. It also informs the other neighboring routers about the changes it has noticed. Route information changes when new networks are connected or old networks are dis- connected from the IP network. New networks appear when a network administrator somewhere installs a new router on the network, perhaps exposing a complete net- work behind that router. Networks can be detached either administratively when a network administrator takes a router away from the network, or because of a link failure - perhaps detaching a whole network behind it. To reduce the risk of a complete network being detached, networks are sometimes connected via multiple links and even through multi- ple routers. A good example is when an enterprise buys Internet services from two ISPs to protect against a case where one fails for some reason or other. When an IP packet arrives at a router, generally the router looks at the destination address of the packet, a