Cisco Routing Table: Classless vs Classful behavior.

There is 2 levels of routes:

Level 1 - routes with the subnet mask less or equal to the classful mask, including:

- quad-zero routes;

- supernet routes;

- classful network routes (can also be a parent route).

Level 2 - a (child) route that is a subnet of a classful network. It is always an ultimate route.

Ultimate route - a route that contains a next-hop IP-address or an exit interface.

Parent route - a route that doesn't contain a next-hop IP-address or an exit interface. It is being automatically created when any subnet of a classful network is added to the routing table.

1 - Level 1 parent route (with classful mask because child routes are variably subnetted).
2 - Level 2 child route.
3 - Level 2 child route.
4 - Level 1 parent route (with non-classful mask because child routes share the same mask).
5 - Level 2 child route.
6 - Level 2 child route.
7 - Level 1 ultimate route.

* At first supernets will be examined and then quad-zero routes.
The classless routing behavior is default since IOS 11.3. To disable it:
R1(config)#no ip classless