Don't understand what the /xx part of an ip address is supposed to means

I thought I did. So if you have an ip address like 70.40.20.15 (just making one up, don't know where it points to), that's supposed to be a class A ip address and the subnet mask should be 255.0.0.0, correct? I would expect the ip/subnet info to then be listed as

70.40.20.15/8

I ask because I have an amazon aws and I try to limit which ip addresses can shell into it. So using the above example, I tried creating a rule with

70.40.20.15/8

but aws said it had to be

70.40.20.15/32

Ok, so after all of my googling clearly I have no clue what the final /xx is supposed to mean. Can anybody clue me in?

That's CIDR notation. 70.40.20.15/8 and 70.235.189.2/8 are the same thing, both representing 70.x.y.z and the netmask 255.0.0.0. AWS might not like the fact that you used actual numbers for the X, Y, and Z (ie, that you had bits set) - try 70.0.0.0/8.

Ok, that worked as well. Thanks for the quick reply. I'll read the link you posted to learn more about this. Thanks for educating.