What is IP routing?

Answer: C. Router C

Of the routes that match the destination address, a router chooses the one with the longest (most specific) prefix. This is called longest prefix match. 10.1.5.9 matches 0.0.0.0/0, 10.0.0.0/8 and 10.1.0.0/16, but it is not in the 10.1.2.0/24 range (10.1.2.0 to 10.1.2.255). The longest of the matches is /16, so the packet goes to router C. 0.0.0.0/0 is the default route that matches every destination and is used only when no other route matches.

Answer: B. Routers exchange route information using routing protocols and automatically switch to another path when a failure makes a route unusable

Static routing means an administrator configures routes by hand. It is simple and predictable, and suits small networks or the default route of a site with only one exit, but it does not switch over automatically after a failure. In dynamic routing, routers exchange information with routing protocols such as OSPF or BGP to build their routing tables automatically, and recalculate another path when a link fails. On the internet, routes between organizations' networks (autonomous systems) are exchanged with BGP. Real networks commonly combine both.