I am attaching my scenario and my "show ip route" and "show running" for my two routers.
I read about Classful Routing: When a packet's destination only matches a router's default route, and does not match any other routes, only use the default route if this router does not know any routes in the classful network in which the destination IP address reside.
I am trying to configure the scenario so that the ping from PC1 to PC2 fails because the routers are configure with the "no ip classless" and both PCs are in the same classful network.
Any idea why my ping is working? I was expecting this wouldn't work. Please help to understand!!
Of course, you're right! The reason your ping is working, is because of CEF (which is usually enabled by default).
CEF does a route lookup in hardware, and apparently "no ip classless" does no apply when CEF is enabled.
For your ping to fail you need to turn off CEF and enable process switching. Try "no ip cef" in global config, and see if that does the trick.