connecting 2 different classes subnet

Hi,

I'm facing some problem during my self study on ip address and subnet mask.

Let say i have and ip address 10.9.30.100/8 and 10.9.31.99/16
the above ip addresses can communicate,

while if the ip address are 10.9.30.100/8 and 10.9.31.99/24
the above ip addresses cannot communicate anymore,

im using a baseline switch 2928 sfp plus to perform this testing.

Can anyone provide me some reference on how to calculate/determine how different subnet can communicating?

Thank you.