64 because 2 bits are needed for the hosts on the network, leaving 6 bits for the subnet.
Classless routing protocols advertise subnet mask information along with the network prefixes. Classful routing protocols do not. Therefore, for a classful protocol, all subnets for the major network number being used must be the same length. Also, classful protocol cannot support discontiguous networks prefixes.
The passive state means that a router has a successor for a route. The active state means that a router does not have a successor or feasible successor for a route and is actively sending queries to neighbors to get information about the route.
The reported distance to a route that is sent to another router is the feasible distance on the reporting router. Feasible distance is the reported distance plus the metric between the receiving and reporting routers. The route with the lowest feasible distance is the successor. Any routes with a reported distance that is less than the feasible distance are feasible successors.