Routing tables and longest-prefix matching

A route is a forwarding instruction

A routing-table entry has a destination prefix and a way to reach it, often a next-hop address or outgoing interface. Connected routes describe attached networks. Static routes are configured by a person; routing protocols learn routes from other routers. A default route, 0.0.0.0/0 for IPv4, matches destinations without a more specific installed route.

To forward a packet, compare its destination with the installed routes and choose the longest matching prefix. A /24 match is more specific than a /16 match. Administrative distance helps select between sources offering the same prefix; metrics compare paths according to the routing protocol. Do not compare every route’s distance before checking its prefix.

Route lookup example

Suppose routes exist for 10.0.0.0/8 via A, 10.1.0.0/16 via B and 10.1.2.0/24 via C. A packet for 10.1.2.50 uses C. A packet for 10.1.9.50 uses B. A return route is also necessary for a successful conversation.

A floating static route has a higher administrative distance than the route it backs up. IPv6 uses the same longest-prefix idea, with ::/0 as the default. Interface-only static routes on multiaccess networks have different implications from next-hop routes; use the exercise’s specified design.

show ip route