Remember that an IPv4 address is 32-bits long, split into four groups of 8 bits each. With CIDR notation, an IP address is followed by a slash and a number that represents the number of bits needed to represent the network portion of the address; the remaining bits would then be assigned to hosts. Therefore, you can always subtract the number at the end of the CIDR notation from 32 to get the number of bits for host assignment. Let's look at a couple examples.

192.168.105.0/24 means that the first 24 bits identify the network. To understand this, let's see 192.168.105.0 in binary:

When assigning addresses in this subnet, we'd only change the final 8 bits, with the highest value, 11111111, being the broadcast address of this subnet:

Calculating netmasks from the CIDR notation and vice versa is easy: whatever bits make up the network portion, turn those into all ones and turn the hosts portion into all zeroes. Then, convert that value into an IP address. That'll be your netmask:

One more example for the road, 10.14.140.0/19: